Use -Wno-missing-declarations with CWARNFLAGS for skein.c

-Wno-missing-variable-declarations is a clang-specific flag,
so gcc (not 4.2.1, in particular 6.3.0 in my case) dies when
it's passed the flag.

X-MFC with: r304321
Reported by: amd64-gcc-6.3.0 (devel/amd64-xtoolchain-gcc)
Sponsored by: Dell EMC Isilon