Makefile: Add -Wmising-prototypes to HOSTCFLAGS

Now that all host programs use static for all private functions and
forward prototypes for all extern functions, add -Wmissing-prototypes to
HOSTCFLAGS in the hopes of keeping it that way.

All versions of GCC supported by the kernel handle -Wmissing-prototypes.

Signed-off-by: Josh Triplett <josh@joshtriplett.org>