Remove unused include_dir

Change-Id: I4fabc860ad586755e28bd579df743822bcc2bf47
Reviewed-by: Andras Becsi <andras.becsi@digia.com>
Use files for install instead of extra

We need to add the no_check_exist value in order for qmake to just accept
that those files will eventually exist.
This has the benefits of not messing with $(INSTALL_ROOT) and also
stripping the library, which could prove useful.

Change-Id: I43c230f185420ba92ff8e86f49b4e838ca5dd917
Reviewed-by: Andras Becsi <andras.becsi@digia.com>