Correct the list of files included in "make dist"

At the very least, include the files named in the sources, like
HEADERS. It was quite surprising to send a tarball that included the
.pro file and the .cpp sources, but none of the headers.

On the other hand, the .qmake.cache file need need not be sent either,
despite being include()d in qmake's processing.

Change-Id: I8f48ca3e8040f954f321f4643b01c0f36aafe2d7
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>