LibGfx: Avoid inclusion of xmmintrin.h on non-X86 architectures

Gamma.h includes xmmintrin.h, which is X86-only. The file contains
code in other places that is compiled conditionally on __SSE__, so the
same macro is used to determine inclusion of the header.

Userland/Libraries/LibGfx/Gamma.h

@@ -8,7 +8,10 @@
 
 #include "Color.h"
 #include <AK/Math.h>
-#include <xmmintrin.h>
+
+#ifdef __SSE__
+#    include <xmmintrin.h>
+#endif
 
 #include <AK/SIMD.h>