S390: Fix build error in iconvdata/bug-iconv11.c.

This fixes the following build error on S390 31bit while building the test
iconvdata/bug-iconv11.c with gcc 5.3:
bug-iconv11.c: In function ‘test_ibm93x’:
bug-iconv11.c:59:11: error: format ‘%td’ expects argument of type ‘ptrdiff_t’, but argument 2 has type ‘size_t {aka long unsigned int}’ [-Werror=format=]
   printf ("  ==> %td: %s\n"
           ^
cc1: all warnings being treated as errors

This patch uses %zu format specifier for argument size_t ret instead of %td.

ChangeLog:

	* iconvdata/bug-iconv11.c (test_ibm93x):
	Use %zu printf format specifier for size_t argument.