Use QStringLiteral instead of QString::fromLatin1

The value is returned, so it would be converted to QString anyway.
This saves the allocation.

Change-Id: I1a4ec4e16bccdd23e662be9b3dd7f8f09b4197ee
Reviewed-by: Christian Ehrlicher <ch.ehrlicher@gmx.de>

src/gui/text/qfont.cpp

@@ -2180,7 +2180,7 @@ void QFont::cacheStatistics()
 */
 QString QFont::lastResortFamily() const
 {
-    return QString::fromLatin1("helvetica");
+    return QStringLiteral("helvetica");
 }
 #endif