libs/gst/check/gstcheck.*: factor out the method from tests that checks size of struc...

libs/gst/check/gstcheck.*: factor out the method from tests that checks size of structures, and add code to generate ...

Original commit message from CVS:
* libs/gst/check/gstcheck.c: (gst_check_abi_list):
* libs/gst/check/gstcheck.h:
factor out the method from tests that checks size of structures,
and add code to generate the header containing these sizes
* tests/check/gst/gstabi.c: (GST_START_TEST):
* tests/check/gst/struct_i386.h:
* tests/check/libs/libsabi.c: (GST_START_TEST):
* tests/check/libs/struct_i386.h:
use it