nativesdk-qtbase.inc: Fix build failure removal of include and mkspecs

The 7d8f8d6 change (nativesdk-qtbase.inc: Remove unused contents)
introduced a build failure that was not catch in my test. This patch
fixes it avoiding removing the headers and mkspecs files which are
need for the nativesdk-qttools recipe.

Signed-off-by: Otavio Salvador <otavio@ossystems.com.br>