Adjust fbt_unload() definition to avoid clang 15 warning
With clang 15, the following -Werror warning is produced:
sys/cddl/dev/fbt/fbt.c:1273:11: error: a function declaration without a prototype is deprecated in all versions of C [-Werror,-Wstrict-prototypes] fbt_unload() ^ void
This is because fbt_unload() is declared with a (void) argument list,
but defined with an empty argument list. Make the definition match the
declaration.
MFC after: 3 days
(cherry picked from commit 6339314c737f44477c1168819353087c2f4816eb)