qwt: fix unpackaged files

* fixes:
qwt-6.0.1: qwt: Files/directories were installed but not shipped
  /usr/features
  /usr/doc
  /usr/plugins
  /usr/features/qwtconfig.pri
  /usr/features/qwtmathml.prf
  /usr/features/qwt.prf
  /usr/plugins/designer
  /usr/plugins/designer/libqwt_designer_plugin.so
  /usr/plugins/designer/.debug
  /usr/plugins/designer/.debug/libqwt_designer_plugin.so

Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>