tst-solaris-taskq.so: Fix printf format string

Spotted by GCC:

../../tests/tst-solaris-taskq.c: In function ‘tq_test_func’:
../../tests/tst-solaris-taskq.c:25:2: error: format ‘%s’ expects argument of type ‘char *’, but argument 3 has type ‘void *’ [-Werror=format=]
  kprintf("%s called for %s\n", __func__, arg);
  ^

Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>