Scalar F32 Sigmoid micro-kernels

Make Sigmoid operator usable in WebAssembly builds

PiperOrigin-RevId: 286927004
diff --git a/src/init.c b/src/init.c
index c5ff109..c01d684 100644
--- a/src/init.c
+++ b/src/init.c
@@ -1412,6 +1412,7 @@
     };
     xnn_params.f32.clamp = (xnn_univector_ukernel_function) xnn_f32_clamp_ukernel__wasm;
     xnn_params.f32.hswish = (xnn_univector_ukernel_function) xnn_f32_hswish_ukernel__wasm_x4;
+    xnn_params.f32.sigmoid = (xnn_univector_ukernel_function) xnn_f32_sigmoid_ukernel__scalar_lut64_p2_div_x2;
     xnn_params.f32.prelu = (struct prelu_parameters) {
       .ukernel = (xnn_prelu_ukernel_function) xnn_f32_prelu_ukernel__wasm_2x4,
       .row_tile = 4,