fix fallback for xcode specs

clearly, it's not very useful if nobody noticed it yet. anyway ...

Change-Id: Ieef1593e42031f8c17a3d7e9e67e3194ded9c066
Reviewed-by: Joerg Bornemann <joerg.bornemann@theqtcompany.com>

mkspecs/features/spec_post.prf

@@ -18,6 +18,7 @@ isEmpty(QMAKE_PLATFORM) {
             TARGET_PLATFORM = win32
         } else:if(equals(MAKEFILE_GENERATOR, PROJECTBUILDER) \
                  |equals(MAKEFILE_GENERATOR, XCODE)) {
+            TARGET_PLATFORM = macx
         } else {
             error("Qmake spec sets an invalid MAKEFILE_GENERATOR.")
         }