FIX: function argument fix for C23 conformance

C23 now regards function declaration without a parameter list
as that with no parameter (i.e. having `(void)`).

Specify parameter list explicitly to make code C23
conformant.

Closes lxde#106 .

Author: mtasaka

src/base/fm-dummy-monitor.c

@@ -37,7 +37,7 @@
 
 G_DEFINE_TYPE(FmDummyMonitor, fm_dummy_monitor, G_TYPE_FILE_MONITOR);
 
-static gboolean cancel()
+static gboolean cancel(GFileMonitor *)
 {
     return TRUE;
 }