discoverer: add casts to avoid warnings in osx builds

discoverer: add casts to avoid warnings in osx builds

Fix the following warnings by adding casts.

gstdiscoverer.c:1801:17: error: format specifies type 'unsigned long' but the argument has type 'off_t' (aka 'long long') [-Werror,-Wformat]
      location, file_status.st_size, file_status.st_mtime);
                ^~~~~~~~~~~~~~~~~~~
gstdiscoverer.c:1801:38: error: format specifies type 'long long' but the argument has type '__darwin_time_t' (aka 'long') [-Werror,-Wformat]
      location, file_status.st_size, file_status.st_mtime);

https://gitlab.freedesktop.org/gstreamer/gst-plugins-base/issues/570