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