Fix stdio-common/tst-fmemopen.c format warnings.

Fix stdio-common/tst-fmemopen.c format warnings.

Testing for 32-bit x86 shows up warnings in
stdio-common/tst-fmemopen.c where off_t values are passed to %zu
printf formats.  Since the values are in messages relating to function
calls where the relevant argument is of type size_t, it seems most
appropriate to cast explicitly to size_t when passing to printf, which
this patch does.

Tested for 32-bit x86.

	* stdio-common/tst-fmemopen.c (do_test): Cast st_size values to
	size_t for %zu format.