Add gavgpool_spchw_scalar__x1 kernel.

PiperOrigin-RevId: 275009343
diff --git a/CMakeLists.txt b/CMakeLists.txt
index 2cb377c..1ea59b6 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -176,6 +176,7 @@
   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