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)