tslog: Move sysinit_tslog_shim() into kern_tslog.c

struct sysinit's func pointer requires its address, thus a real function
is generated in every translation unit when the source file has SYSINITs
declared. That results in plenty of identical sysinit_tslog_shim in the
final kernel file, in which only one is used and others are left useless.

MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D52413

(cherry picked from commit 5f9a05e574050c8b3f6b444311a12e8fb293ad1a)
(cherry picked from commit bd721f6e8040f0b9d0546c293954290844b6a070)
(cherry picked from commit 112d1a5f5df0edb1e530348143413407b2659325)