For consistency with what 38cd1d9a "clew: move
this foreign code to external/clew" did for clew.c.  Library_scfilt depends on
clew/clew.h, simply make it depend on the full clew external.

Change-Id: Idcbf56a69084e108832c8970507ebea0f7ff400b
Reviewed-on: https://gerrit.libreoffice.org/24035Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Stephan Bergmann <sbergman@redhat.com>