lindebugfs: Export symbols

We currently rely on the kernel linker resolving undefined references
against local symbols from other kernel modules. Be explicit about the
symbols to export, in advance of changing that behaviour.

PR: 207898
Reviewed by: kib, bz
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D47980

(cherry picked from commit c3d2c959b5c0ea05d6e5162def35d51ec267590c)