support for private depends in modules

support for private depends in modules

if a module's private headers add additional dependencies, QT_PRIVATE is
now the place to declare them. note however, that this may not contain
other private headers in turn - that would be much harder to implement,
and we want the explicitness anyway.

Change-Id: Ic516fcf1a003c95798df4fbe216f92016afaf47e
Reviewed-by: Joerg Bornemann <joerg.bornemann@nokia.com>
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@nokia.com>