oox: fix spurious rebuilds of namespacemap.cxx

There is a missing dependency on one of the generated header files.

Change-Id: Id3fa2ccf2c2a31527b795861afdf557882f56972
(cherry picked from commit 63e6cbc3180047000787a9071513152fdd9ce942)
Reviewed-on: https://gerrit.libreoffice.org/11844
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>