distroquery_functions.cpp: fix compilter type warning

src/distroquery_functions.cpp

@@ -148,7 +148,7 @@ string urlDecode(string str) {
     char ch;
     unsigned int ii;
 
-    for (int i=0; i < str.length(); i++){
+    for (size_t i=0; i < str.length(); i++) {
         if (str[i] != '%') {
             if (str[i] == '+')
                 ret += ' ';