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);

gstreamer/gst-plugins-base#570
8 jobs for issue570 in 30 minutes and 12 seconds (queued for 33 seconds)
latest
Status Job ID Name Coverage
  Preparation
passed #185556
gst indent

00:00:44

passed #185555
manifest

00:00:44

 
  Build
passed #185557
build fedora x86_64

00:05:47

 
  Test
passed #185558
check fedora

00:03:57

 
  Full Builds
passed #185560
build cerbero android universal

00:19:27

passed #185561
build cerbero cross win32

00:11:36

passed #185562
build cerbero cross win64

00:06:39

passed #185559
build cerbero fedora x86_64

00:07:45