[X86] Undef __DEFAULT_FN_ATTRS in avx512fintrin.h.

Fixes test failure after r340713

llvm-svn: 340714
diff --git a/clang/lib/Headers/avx512fintrin.h b/clang/lib/Headers/avx512fintrin.h
index 7c65a1a..86425b4 100644
--- a/clang/lib/Headers/avx512fintrin.h
+++ b/clang/lib/Headers/avx512fintrin.h
@@ -9595,5 +9595,6 @@
 
 #undef __DEFAULT_FN_ATTRS512
 #undef __DEFAULT_FN_ATTRS128
+#undef __DEFAULT_FN_ATTRS
 
 #endif /* __AVX512FINTRIN_H */