Exclude non-supported imageformats for WinRT

Change-Id: Icaf64ef0b949e75402c1440826f24e42de96df62
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>

src/plugins/imageformats/imageformats.pro

@@ -6,3 +6,8 @@ SUBDIRS = \
     tiff \
     dds \
     icns
+
+winrt {
+    SUBDIRS -= tiff \
+               tga
+}