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: NJosh Triplett <josh@joshtriplett.org>

Makefile

@@ -221,7 +221,7 @@ CONFIG_SHELL := $(shell if [ -x "$$BASH" ]; then echo $$BASH; \
 
 HOSTCC       = gcc
 HOSTCXX      = g++
-HOSTCFLAGS   = -Wall -Wstrict-prototypes -O2 -fomit-frame-pointer
+HOSTCFLAGS   = -Wall -Wmissing-prototypes -Wstrict-prototypes -O2 -fomit-frame-pointer
 HOSTCXXFLAGS = -O2
 
 # Decide whether to build built-in, modular, or both.