Autogen: Continue search for FOO_p.h when FOO.h was found

Sebastian Holtermannrequested to merge
sebholt/cmake:autogen_p into master

When searching for headers for a source file FOO.cpp, AUTOMOC stops when FOO.h was found. This differs from previous version where the search continued to see if FOO_p.h exists as well.

This patch restores the old behavior.

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