fix void* arithmetic compilation warning

Signed-off-by: Loic Dachary <loic@dachary.org>
(cherry picked from commit ce61fb053da692477877dc2370c637efb4df43ff)
do not compile if used in SSE4 code path only

Acknowledge that gf_w128_split_4_128_multiply_region and
gf_w128_split_4_128_sse_multiply_region are only used when the
INTEL_SSE4 flag is present, even though they only need INTEL_SSSE3

It suppresses a compilation warning complaining about them not being
used if INTEL_SSE4 is absent and INTEL_SSSE3 is present.

Signed-off-by: Loic Dachary <loic@dachary.org>
(cherry picked from commit d569220629d476d687859968fc4bee3194eca16f)