aesni(4): Fix GCC build

The GCC xmmintrin.h header brokenly includes mm_malloc.h unconditionally.
(The Clang version of xmmintrin.h only includes mm_malloc.h if not compiling
in standalone mode.)

Hack around GCC's broken header by defining the include guard macro ahead of
including xmmintrin.h.

Reported by: lwhsu, jhb
Tested by: lwhsu
Sponsored by: Dell EMC Isilon