Fixed 'make docs' lauching of qhelpgenerator

It assumed qhelpgenerator was in the path, though this isn't necessarily
the case. Use the absolute directory instead.

doc/doc.pri

 unix {
     QDOC = SRCDIR=$$PWD OUTDIR=$$OUT_PWD/doc/html $$(QTDIR)/tools/qdoc3/qdoc3
-    HELPGENERATOR = qhelpgenerator
+    HELPGENERATOR = $$(QTDIR)/bin/qhelpgenerator
 } else {
     QDOC = $$(QTDIR)\tools\qdoc3\release\qdoc3.exe
-    HELPGENERATOR = qhelpgenerator
+    HELPGENERATOR = $$(QTDIR)\bin\qhelpgenerator.exe
 }
 
 QHP_FILE = $$OUT_PWD/doc/html/qtcreator.qhp