s390x: Make code compilable with -Wmissing-prototypes and -Wstrict-prototypes

Include the right header in mmu.c to make sure that the implementations
match the prototypes, and avoid old-style declaration in float.h.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Message-Id: <1538123582-17442-4-git-send-email-thuth@redhat.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>