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

sys/geom/geom_io.c

@@ -269,7 +269,7 @@ g_reset_bio(struct bio *bp)
 }
 
 void
-g_io_init()
+g_io_init(void)
 {
 
 	g_bioq_init(&g_bio_run_down);