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 47bfbc9026 as
it causes problems depending on the include path. The proper
fix will be to ensure that cpp/src is included before gpgme/src.

lang/qt/src/threadedjobmixin.h

@@ -41,7 +41,7 @@
 #include <QIODevice>
 
 #ifdef BUILDING_QGPGME
-# include "lang/cpp/src/context.h"
+# include "context.h"
 # include "interfaces/progressprovider.h"
 #else
 # include <gpgme++/context.h>