gst/gstutils.h: Try to fix 'dereferencing type-punned pointer will break strict aliasing rules' warnings with C++ com...

Original commit message from CVS:
* gst/gstutils.h: (GST_BOILERPLATE_FULL):
Try to fix 'dereferencing type-punned pointer will break strict
aliasing rules' warnings with C++ compilers and GLib >= 2.14.0: GLib
changed the default GType typedef from gulong to gsize at some point,
but kept GType typedef'ed to gulong for C++ for ABI reasons; the
g_once_* functions all take a gsize * though, so work around the type
mismatch for C++ by doing everything in gsize and casting to GType
later.