s390/compat: select COMPAT_BINFMT_ELF only if BINFMT_ELF is set

Fix this:
warning: (COMPAT) selects COMPAT_BINFMT_ELF which has unmet
         direct dependencies (COMPAT && BINFMT_ELF)

Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>