bld/makdep.c: add a return type to main()

[harshula]

Fixes build failure with [email protected]

cc -o makdep /tmp/root/spack-stage/spack-stage-cice5-git.2023.10.19_access-om2-q26rhh7gzs7pxowlbybq555kzxij6qf2/spack-src/bld/makdep.c
/tmp/root/spack-stage/spack-stage-cice5-git.2023.10.19_access-om2-q26rhh7gzs7pxowlbybq555kzxij6qf2/spack-src/bld/makdep.c:54:1: error: type specifier missing, defaults to 'int'; ISO C99 and later do not support implicit int [-Wimplicit-int]
   54 | main (int argc, char **argv)
      | ^
      | int
1 error generated.
exit 2

[anton-seaice]

Thanks @harshula