metal : update support condition for im2col + fix warning (llama/0)

ggml/src/ggml-metal.m

@@ -799,8 +799,9 @@ static bool ggml_metal_supports_op(const struct ggml_backend_metal_context * ctx
             return ctx->support_simdgroup_reduction;
         case GGML_OP_NORM:
         case GGML_OP_ROPE:
-        case GGML_OP_IM2COL:
             return true;
+        case GGML_OP_IM2COL:
+            return op->src[0]->type == GGML_TYPE_F16;
         case GGML_OP_POOL_1D:
         case GGML_OP_POOL_2D:
             return false;