maliit-framework-qt5: package only ${libdir}/qt5/mkspecs within ${PN}-dev

When we're building maliit with it's input context plugin enable it gets installed into
${libdir}/qt5/plugins/platforminputcontexts which couldn't be added to ${PN} cause it's
already in ${PN}-dev and the only part which should go into ${PN}-dev is
${libdir}/qt5/mksepcs and nothing else from ${libdir}/qt5.

Signed-off-by: Simon Busch <morphis@gravedo.de>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>