append $$QMAKE_DEFAULT_INCDIRS to moc invocations

unlike the real compiler, moc does not have these directories built in,
so it would not find headers from a system install of qt.

Task-number: QTBUG-28870
Change-Id: I86f18cdc8953145190163746dae59f4e784f2d78
Reviewed-by: Joerg Bornemann <joerg.bornemann@digia.com>

mkspecs/features/moc.prf

@@ -47,7 +47,7 @@ defineReplace(mocCmdBase) {
     } else {
         incvar = $(INCPATH)
     }
-    RET += $$QMAKE_MOC $(DEFINES) $$incvar $$join(QMAKE_COMPILER_DEFINES, " -D", -D) $$QMAKE_MOC_OPTIONS
+    RET += $$QMAKE_MOC $(DEFINES) $$join(QMAKE_COMPILER_DEFINES, " -D", -D) $$incvar $$join(QMAKE_DEFAULT_INCDIRS, " -I", -I) $$QMAKE_MOC_OPTIONS
     return($$RET)
 }