misc: Fix various compiler warnings on MinGW

gstharness.c: Use G_GSIZE_FORMAT instead of hard-coding %zu
error: unknown conversion type character 'z' in format [-Werror=format]

gst-inspect.c: GPid is void* on non-UNIX, and we only use it on UNIX
error: initialization makes pointer from integer without a cast [-Werror]

gstmeta.c: Use and then discard value
error: value computed is not used [-Werror=unused-value]

With this, gstreamer builds with -Werror on MinGW
8 jobs for fix-mingw-warnings in 29 minutes and 13 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Preparation
passed #99533
gst indent

00:00:37

passed #99532
manifest

00:00:19

 
  Build
passed #99534
build fedora x86_64

00:04:30

 
  Test
passed #99535
check fedora

00:03:42

 
  Full Builds
passed #99536
build android arm64 api28

00:03:07

passed #99538
build cerbero cross win32

00:09:07

passed #99539
build cerbero cross win64

00:06:20

passed #99537
build cerbero fedora x86_64

00:06:27