[RFC] adding -Wmissing-include-dirs to CWARNFLAGS

hi there,

i'd like to propose adding -Wmissing-include-dirs to CWARNFLAGS. this will let
tinderbox fail, if any new kernel code was committed with (a) broken include
dir(s).

i ran a test via

make toolchains
make MAKE_JUST_KERNELS=yes tinderbox

and nothing seemed to go wrong with the extra warning enabled. i even found a
missing include dir, which should be fixed by the attached patch.

opinions?

so far i've only tested this with gcc. i think someone on #freebsd-clang told
me that -Wmissing-include-dirs is a noop for clang (for whatever reasons).

cheers.
alex

-- 
a13x