Fix Qt build when mtdev is detected

If it's detected, we have "mtdev" in QT_CONFIG, not in CONFIG. With
the bad test, libQt5PlatformSupport.prl would not get -lmtdev and, in
turn, the evdevtouch generic plugin would fail to link.

Change-Id: I5dab57b648e66943f98a22527717a20be35f02a4
Reviewed-by: Richard J. Moore <rich@kde.org>
Reviewed-by: Robin Burchell <robin+qt@viroteck.net>

src/platformsupport/input/evdevtouch/evdevtouch.pri

@@ -8,7 +8,7 @@ contains(QT_CONFIG, libudev) {
     LIBS_PRIVATE += $$QMAKE_LIBS_LIBUDEV
 }
 
-contains(CONFIG, mtdev) {
+contains(QT_CONFIG, mtdev) {
     CONFIG += link_pkgconfig
     PKGCONFIG_PRIVATE += mtdev
 }