Polyfill missing _cvtu32_mask16 intrinsic on old gcc

PiperOrigin-RevId: 284006031
diff --git a/src/f32-gemm/avx512-broadcast.c.in b/src/f32-gemm/avx512-broadcast.c.in
index 4e69c17..807e97d 100644
--- a/src/f32-gemm/avx512-broadcast.c.in
+++ b/src/f32-gemm/avx512-broadcast.c.in
@@ -10,6 +10,7 @@
 #include <immintrin.h>
 
 #include <xnnpack/gemm.h>
+#include <xnnpack/intrinsics-polyfill.h>
 
 
 void xnn_f32_gemm${"inc" if INC else ""}_ukernel_${MR}x${NR}__avx512f_broadcast(