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