unbound: Fix config.h

FreeBSD src does not support HAVE_DECL_EVSIGNAL_ASSIGN. While reviewing
the new config.h after regenerating it, this definition was not removed.

Updating config.h is a manual process of configuring the port and
copying/merging the generated config.h into src. This definition was
missed and not removed (#undef'd).

Fixes: 1838dec31895fd4752fa8631322ab93be0705a66

(cherry picked from commit 06b93ef8cda227d755d7f6cfa95d08c0ccc9dc2d)