tests: posix: xsi_single_process: reduce min ram to 16 kiB

The `qemu_cortex_m0/nrf51822` platform only has 16 kiB of RAM. However,
`qemu_cortex_m0` is listed as one of the integration platforms for this
testsuite and the `newlib` test configuration listed a `min_ram` of 24.

Decrease the minimum ram for that platform (universally) down to 16 kiB
so that all of the test configurations can run, thus avoiding the false
positive error condition in twister.

Signed-off-by: Chris Friedt <cfriedt@tenstorrent.com>