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

tools/testing/selftests/powerpc/mm/.gitignore

 # SPDX-License-Identifier: GPL-2.0-only
+bad_accesses
+exec_prot
 hugetlb_vs_thp_test
-subpage_prot
-tempfile
-prot_sao
-segv_errors
-wild_bctr
 large_vm_fork_separation
-bad_accesses
-tlbie_test
+large_vm_gpr_corruption
 pkey_exec_prot
 pkey_siginfo
+prot_sao
+segv_errors
 stack_expansion_ldst
 stack_expansion_signal
-large_vm_gpr_corruption
+subpage_prot
+tempfile
+tlbie_test
+wild_bctr