test/javacompat: get rid of a compiler warning

src/test/javacompat.cpp

@@ -87,7 +87,7 @@ int main(){
 
     std::vector<uint8_t> data = JavaCompat::exportJavaString(normalString).value;
     std::string exportedString = std::string();
-    for(int i=0; i<data.size(); i++){
+    for(int i=0; i<(int) data.size(); i++){
         exportedString.push_back(data[i]);
     }