Adjust fbd_list() definition to avoid clang 15 warning

With clang 15, the following -Werror warnings is produced:

sys/dev/fb/fbd.c:205:9: error: a function declaration without a prototype is deprecated in all versions of C [-Werror,-Wstrict-prototypes]
fbd_list()
        ^
         void

This is because fbd_list() is declared with a (void) argument list, but
defined with an empty argument list. Make the definition match the
declaration.

MFC after: 3 days