s390/module: fix building test_modules_helpers.o with clang

Move test_modules_return_* prototypes into a header file in order to
placate -Wmissing-prototypes.

Fixes: 90c53187

 ("s390/module: test loading modules with a lot of relocations")
Reported-by: kernel test robot <lkp@intel.com>
Reviewed-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>