Fixed compile warning for clang linux onnx2ncnn.cpp [-Wformat] (#2185)

tools/onnx/onnx2ncnn.cpp

@@ -3775,7 +3775,7 @@ int main(int argc, char** argv)
                 }
                 else if (attr.type() == 2)
                 {
-                    fprintf(stderr, "  # %s=%lld\n", attr.name().c_str(), attr.i());
+                    fprintf(stderr, "  # %s=%lld\n", attr.name().c_str(), (long long)attr.i());
                 }
                 else if (attr.type() == 3)
                 {