C23 conformance with prototype with no parameter

[mtasaka]

(developing) gcc15 now defaults to -std=gnu23 (C23) and libfm https://github.com/lxde/libfm/tree/2d94c3c8c4a2c4e252a79da3ef2b6a70e9ec37e1 fails to compile with C23:

libtool: compile:  gcc -DHAVE_CONFIG_H -I. -I/builddir/build/BUILD/libfm-1.3.2_20250109git2d94c3c8-build/libfm-1.3.2-20250109git2d94c3c8/libfm/_BUILDDIR_gtk2/../src -I.. -I/builddir/build/BUILD/libfm-1.3.2_20250109git2d94c3c8-build/libfm-1.3.2-20250109git2d94c3c8/libfm/_BUILDDIR_gtk2/../src/base -I/builddir/build/BUILD/libfm-1.3.2_20250109git2d94c3c8-build/libfm-1.3.2-20250109git2d94c3c8/libfm/_BUILDDIR_gtk2/../src/job -DPACKAGE_LOCALE_DIR=\"/usr/share/locale\" -Werror-implicit-function-declaration -D_LARGEFILE_SOURCE -D_REENTRANT -D_FILE_OFFSET_BITS=64 -DG_DISABLE_ASSERT -DG_DISABLE_CHECKS -DG_DISABLE_CAST_CHECKS -I/usr/include/gio-unix-2.0 -DWITH_GZFILEOP -I/usr/include/libmount -I/usr/include/blkid -I/usr/include/glib-2.0 -I/usr/lib64/glib-2.0/include -I/usr/include/sysprof-6 -pthread -I/usr/include/menu-cache -I/usr/include/glib-2.0 -I/usr/lib64/glib-2.0/include -I/usr/include/sysprof-6 -pthread -I/usr/include/dbus-1.0 -I/usr/lib64/pkgconfig/../../include/dbus-1.0 -I/usr/lib64/pkgconfig/../../lib64/dbus-1.0/include -I/usr/include/glib-2.0 -I/usr/lib64/glib-2.0/include -I/usr/include/sysprof-6 -pthread -DPACKAGE_DATA_DIR=\"/usr/share/libfm\" -DPACKAGE_MODULES_DIR=\"/usr/lib64/libfm/modules\" -O2 -flto=auto -ffat-lto-objects -fexceptions -g -grecord-gcc-switches -pipe -Wall -Werror=format-security -Wp,-U_FORTIFY_SOURCE,-D_FORTIFY_SOURCE=3 -Wp,-D_GLIBCXX_ASSERTIONS -specs=/usr/lib/rpm/redhat/redhat-hardened-cc1 -fstack-protector-strong -specs=/usr/lib/rpm/redhat/redhat-annobin-cc1 -m64 -march=x86-64 -mtune=generic -fasynchronous-unwind-tables -fstack-clash-protection -fcf-protection -mtls-dialect=gnu2 -fno-omit-frame-pointer -mno-omit-leaf-frame-pointer -c /builddir/build/BUILD/libfm-1.3.2_20250109git2d94c3c8-build/libfm-1.3.2-20250109git2d94c3c8/libfm/_BUILDDIR_gtk2/../src/base/fm-dummy-monitor.c  -fPIC -DPIC -o base/.libs/libfm_la-fm-dummy-monitor.o
/builddir/build/BUILD/libfm-1.3.2_20250109git2d94c3c8-build/libfm-1.3.2-20250109git2d94c3c8/libfm/_BUILDDIR_gtk2/../src/base/fm-dummy-monitor.c: In function 'fm_dummy_monitor_class_init':
/builddir/build/BUILD/libfm-1.3.2_20250109git2d94c3c8-build/libfm-1.3.2-20250109git2d94c3c8/libfm/_BUILDDIR_gtk2/../src/base/fm-dummy-monitor.c:48:22: error: assignment to 'gboolean (*)(GFileMonitor *)' {aka 'int (*)(struct _GFileMonitor *)'} from incompatible pointer type 'gboolean (*)(void)' {aka 'int (*)(void)'} [-Wincompatible-pointer-types]
   48 |     fm_class->cancel = cancel;
      |                      ^
make[4]: *** [Makefile:2482: base/libfm_la-fm-dummy-monitor.lo] Error 1