Adjust linux_get_char_devices() definition to avoid clang 15 warning

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

sys/compat/linux/linux_util.c:243:23: error: a function declaration without a prototype is deprecated in all versions of C [-Werror,-Wstrict-prototypes]
linux_get_char_devices()
                      ^
                       void

This is because linux_get_char_devices() 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 e90d1b57487759e076b16cd3903a7b69565452b9)