powerpc/selftests: sort mm/.gitignore, add exec_prot

Add exec_prot to to mm/.gitignore and sort the result.

Signed-off-by: Nathan Lynch <nathanl@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20230817-powerpc-selftest-misc-v1-1-a84cc1ef78b2@linux.ibm.com