-Wmissing-prototypes is a useful warning, so add it to the build.

With this we can detect conflicting function prototypes. When a file
implements a function but doesn't include the header file which
provides the prototype for it then conflicting prototypes would go
unnoticed without this warning.

MIPS already had that warning, so we can remove it from the MIPS
Makefile.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>