Quick fix for build with -march that enables avx512f.

This is not where ncnn would include this though, it seems to have
tendency to put headers not where they're used by in every files
before including this.
--- a/src/layer/x86/gemm_int8.h
+++ b/src/layer/x86/gemm_int8.h
@@ -13,4 +13,8 @@
 // specific language governing permissions and limitations under the License.
 
+#if __AVX512F__
+#include "avx512_mathfun.h"
+#endif
+
 #if NCNN_RUNTIME_CPU && NCNN_AVX512VNNI && __AVX512F__ && !__AVX512VNNI__
 void pack_A_tile_int8_avx512vnni(const Mat& A, Mat& AT, int i, int max_ii, int k, int max_kk);