Creates and installs pkg-config .pc file

I was able to build the examples/mimetypefinder example with:
g++ `pkg-config --cflags --libs QtMimeTypes` -o mimetypefinder main.cpp

Change-Id: Ie783f88b74119392be633a6094ba40d16209a7c3
Signed-off-by: Luís Pereira <luis.artur.pereira@gmail.com>
Reviewed-by: David Faure (KDE) <faure@kde.org>