CMake: Treat Qt plugins as MODULE libraries

CMake will complain if you try to use a plugin in target_link_libraries,
and it won't produce import libraries on windows, or use .so.number
on Linux.

Change-Id: I6f0cf8267b3c0e6e5c888703596afe59b3a39141
Reviewed-by: Alexandru Croitor <alexandru.croitor@qt.io>

cmake/QtBuild.cmake

@@ -1368,7 +1368,7 @@ function(add_qt_plugin target)
     if(arg_STATIC)
         add_library("${target}" STATIC)
     else()
-        add_library("${target}")
+        add_library("${target}" MODULE)
     endif()
     qt_internal_add_target_aliases("${target}")