folly/container/detail/F14Table.h: add missing `<emmintrin.h>` header

Summary:
This is the header which provides `_mm_set1_epi8`
(https://software.intel.com/sites/landingpage/IntrinsicsGuide/#text=_mm_set1_epi8).

Although this header is transitively found via other headers (which is why this
isn't a build failure), without this explicit include IWYU's heursitics around
template functions wrongly assume that this header should be provided by
downstream includers of this header.

Reviewed By: yfeldblum

Differential Revision: D18753275

fbshipit-source-id: 7d6ee934542867f2d1e84d40397f48ea8146facc

folly/container/detail/F14Table.h

@@ -76,6 +76,7 @@
 #if FOLLY_NEON
 #include <arm_neon.h> // uint8x16t intrinsics
 #else // SSE2
+#include <emmintrin.h> // _mm_set1_epi8
 #include <immintrin.h> // __m128i intrinsics
 #include <xmmintrin.h> // _mm_prefetch
 #endif