subsys/rtio: Add libc partition for usermode API test

When a libc partition exists, all user mode threads will need access to
libc variables. Add the libc partition in this test case to allow that.

This was detected by running the test on ARM32 with thread local storage
enabled as that uses z_arm_tls_ptr which is included in the libc partition.

Signed-off-by: Keith Packard <keithp@keithp.com>