Adjust function definition in geom_subr.c to avoid clang 15 warnings

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

sys/geom/geom_subr.c:484:16: error: a function declaration without a prototype is deprecated in all versions of C [-Werror,-Wstrict-prototypes]
g_wither_washer()
               ^
                void

This is because g_wither_washer() 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 bfa9298765167bde0eacc538cc5a5bae4cdd7ed3)