include the immintrin.h header only when needed (#1605)
include the immintrin.h header only when needed (#1605)

The immintrin.h header is required for the implementation of the
popcount and coutr_zero function only when the Intel compiler is used.
Move the inclusion accordingly. The current state breaks build on
platforms that don't provide the header, but provide the alternative
means (gcc/clang built-ins, etc).

Signed-off-by: Dan Horák <2591e5f46f28d303f9dc027d475a5c60d8dea17a@danny.cz>
lgritz/OpenShadingLanguage,lgritz/OpenShadingLanguage,lgritz/OpenShadingLanguage,lgritz/OpenShadingLanguage,lgritz/OpenShadingLanguage