Add gavgpool_spchw_scalar__x1 kernel.

PiperOrigin-RevId: 275009343
diff --git a/BUILD b/BUILD
index 54316c1..80c0d9e 100644
--- a/BUILD
+++ b/BUILD
@@ -87,6 +87,7 @@
     "src/f32-dwconv/up1x9-scalar.c",
     "src/f32-dwconv-spchw/3x3p1-scalar.c",
     "src/f32-dwconv-spchw/3x3s2p1-scalar.c",
+    "src/f32-gavgpool-spchw/scalar-x1.c",
     "src/f32-gavgpool/mp7p7q-scalar.c",
     "src/f32-gavgpool/up7-scalar.c",
     "src/f32-gemm/1x4-scalar.c",