Fixes declarative_mimetypes lib install directory

Just install it to the $$[QT_INSTALL_IMPORTS] directory. Qt will search
there.
maemo5 "special" install directory dropped.

Change-Id: I307da5b73d02b665d221d1067a6e83b13b00915c
Signed-off-by: Luís Pereira <luis.artur.pereira@gmail.com>
Reviewed-by: David Faure <david.faure@kdab.com>