fix doc build path

Task-number: QTBUG-34456
Change-Id: I8b2207e9419bda7fb95785f26370e451248e3ee2
Reviewed-by: Topi Reiniö <topi.reinio@digia.com>
Reviewed-by: Jerome Pasion <jerome.pasion@digia.com>

src/activeqt/axshared.prx

@@ -21,3 +21,4 @@ SOURCES = \
 
 MODULE = axbase
 load(qt_module)
+QMAKE_DOCS_TARGETDIR =   # Make qt_docs.prf default to activeqt instead of qtaxbase