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

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

sys/geom/geom_io.c:272:10: error: a function declaration without a prototype is deprecated in all versions of C [-Werror,-Wstrict-prototypes]
g_io_init()
         ^
          void

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

MFC after: 3 days