qfactoryloader: static qt compile fix

statically linked qt does not compile, as versionKeyLiteral() is unused,
causing clang to complain:
error: unused function 'versionKeyLiteral' [-Werror,-Wunused-function]

Change-Id: I6a233081e7c58fce75ece82616f937f29a23a81b
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@theqtcompany.com>

src/corelib/plugin/qfactoryloader.cpp

@@ -58,7 +58,9 @@ namespace {
 
 // avoid duplicate QStringLiteral data:
 inline QString iidKeyLiteral() { return QStringLiteral("IID"); }
+#ifdef QT_SHARED
 inline QString versionKeyLiteral() { return QStringLiteral("version"); }
+#endif
 inline QString metaDataKeyLiteral() { return QStringLiteral("MetaData"); }
 inline QString keysKeyLiteral() { return QStringLiteral("Keys"); }