Prepare for upcoming compile breakage

Qt 5.9 is going to break source compatibility by making the
QStringRef(QString *) constructor explicit.

Since the use of that constructor rather looks unintentional
here, replace its use.

Change-Id: I6f5f2e79f7d92f6e80a1ea0aa12126d9c88b499e
Reviewed-by: Jochen Becher <jochen_becher@gmx.de>
Reviewed-by: Eike Ziller <eike.ziller@qt.io>

src/libs/utils/stringutils.cpp

@@ -183,7 +183,7 @@ bool AbstractMacroExpander::expandNestedMacros(const QString &str, int *pos, QSt
             if (!expandNestedMacros(str, &i, ret))
                 return false;
             varName.chop(1);
-            varName += ret;
+            varName += *ret;
         } else if (currArg == &varName && c == '-' && prev == ':' && validateVarName(varName)) {
             varName.chop(1);
             currArg = &defaultValue;