qt5: small optimization to UnicodeParsedString

by using QString::reserve it should be faster and use less memory
diff --git a/qt5/src/poppler-private.cc b/qt5/src/poppler-private.cc
index b284a74..719ccff 100644
--- a/qt5/src/poppler-private.cc
+++ b/qt5/src/poppler-private.cc
@@ -129,6 +129,7 @@
         }
 
         QString result;
+        result.reserve(stringLength / 2);
         // i = 2 to skip the unicode marker
         for ( int i = 2; i < stringLength; i += 2 )
         {