Disable webp also for ios

Already disabled for android and winrt; seems that libwebp's use of
neon does not compile.

Change-Id: Ia119244da1ae3c92f69dfe5ed91d20e665102958
Reviewed-by: Richard Moe Gustavsen <richard.gustavsen@digia.com>

src/plugins/imageformats/imageformats.pro

@@ -16,4 +16,4 @@ winrt {
                tga
 }
 
-winrt|android: SUBDIRS -= webp
+winrt|android|ios: SUBDIRS -= webp