[core][Qt] Use Qt for UTF16, since codecvt is not always available

platform/default/utf.cpp

@@ -0,0 +1,16 @@
+#include <mbgl/util/utf.hpp>
+
+#include <memory>
+#include <locale>
+#include <codecvt>
+
+namespace mbgl {
+namespace util {
+
+std::u16string utf8_to_utf16::convert(std::string const& utf8) {
+    std::wstring_convert<std::codecvt_utf8_utf16<char16_t>, char16_t> converter;
+    return converter.from_bytes(utf8);
+}
+
+} // namespace util
+} // namespace mbgl