mmx: fix macro name so mmintrin.h is included when desired
I had SIMDE_X86_MMX_NATIVE instead of SIMDE_MMX_NATIVE.  It may not
have been a bad idea to include the architecture in the name, but it
definitely isn't worth it to change now.
@@ -38,7 +38,7 @@ SIMDE_DISABLE_UNWANTED_DIAGNOSTICS
# define SIMDE_MMX_NEON
# endif
-# if defined(SIMDE_X86_MMX_NATIVE)
+# if defined(SIMDE_MMX_NATIVE)
# define SIMDE_MMX_USE_NATIVE_TYPE
# elif !defined(SIMDE_ENABLE_NATIVE_ALIASES) && (defined(SIMDE_ARCH_X86_SSE) || defined(SIMDE_ARCH_X86_SSE2))
# define SIMDE_MMX_USE_NATIVE_TYPE
