boost.simd: multiplies - work around warning in apple's clang:

clang's sse2 headers implement _mm_slli_si128 as macro, which causes some
warnings to fire regarding shadowed variables.

Signed-off-by: Tim Blechmann <tim@klingt.org>
Merge pull request #570 from timblechmann/topic/silence_clang_warning

boost.simd: multiplies - work around warning in apple's clang: