Fix up module unload for syscall_module_handler consumers.

After r273707 it was registering syscalls as static.

This fixes hwpmc module unload.

Reported by: markj


git-svn-id: svn+ssh://svn.freebsd.org/base/head@273953 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f