package/brltty: add host-gawk dependency

host-gawk is used by brltty to build brlapi_constants.h, the build will
fail if mawk is used instead of gawk

Fixes:
 - http://autobuild.buildroot.org/results/4c77f03cbc7ab9e5ae9f24fe6eead1d76c50c743



Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>