[core] only include <sstream> when we need it

include/mbgl/util/string.hpp

@@ -1,6 +1,5 @@
 #pragma once
 
-#include <sstream>
 #include <string>
 #include <cassert>
 #include <cstdlib>
@@ -9,6 +8,9 @@
 // Polyfill needed by Qt when building for Android with GCC
 #if defined(__ANDROID__) && defined(__GLIBCXX__)
 
+// TODO: remove use std::to_string to avoid using <sstream>
+#include <sstream>
+
 namespace std {
 
 template <typename T>