Makefile: Compile with -Wmissing-prototypes and -Wstrict-prototypes

With these compiler flags, we can be sure that the implementation really
matches the prototypes in the headers.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Message-Id: <1538123582-17442-5-git-send-email-thuth@redhat.com>
Acked-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>