[Libreoffice] Do not use path that depend on the repo layout!!!

[Norbert Thiebaud]

http://cgit.freedesktop.org/libreoffice/writer/commit/?id=eae3537858e5c96e501ac62d43a0102386e6c295

this commit breaks the build after onegit conversion because
+
+#include "../../../../../libs-core/sfx2/inc/sfx2/childwin.hxx"
+
does not resolve to a valid location anymore.

so use:
#include <sfx2/childwin.hxx>


Norbert