Revert "Qt: More robust lookup of Cpp's context.h"

* lang/qt/src/threadedjobmixin.h: Revert using full path
for context.h

This reverts commit 47bfbc9026b49b9918359af5fcc1621aab0d1065 as
it causes problems depending on the include path. The proper
fix will be to ensure that cpp/src is included before gpgme/src.