gst/rtp/gstrtph263pay.c: Fix up header structure so that compilers don't add padding...

gst/rtp/gstrtph263pay.c: Fix up header structure so that compilers don't add padding between the structure fields, si...

Original commit message from CVS:
Patch by: Haakon Sporsheim  <haakon.sporsheim at tandberg com>
* gst/rtp/gstrtph263pay.c:
Fix up header structure so that compilers don't add padding
between the structure fields, since that would lead to us
sending RTP packets with broken headers (as is currently the
case when compiling with MSVC). Also see similar fixes in
libgstrtp in gst-plugins-base. (#474616; #471194)