tools/testing/selftests/mm: add missing .gitignore entries

Commit 2bee308f3a ("selftests/mm: use pattern matching in .gitignore")
switched to a pattern-matching mechanism to reduce churn in .gitignore.

It however accidentally excluded the page_frag test's-generated module
intermediate C file with .mod.c extension, and also the local_config.h
header generated if liburing is available locally.

Explicitly fix both the issues, fixing the module-generated C file as a
general pattern as these are always intermediate files that should be
ignored.

Since this is a trivial .gitignore change it doesn't seem necessary to
treat it as a hotfix.

Link: https://lore.kernel.org/20260831-fix-mm-selftests-gitignore-v1-1-c984bbd4c5e4@kernel.org
Signed-off-by: Lorenzo Stoakes (ARM) <ljs@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Reviewed-by: Gregory Price (Meta) <gourry@gourry.net>
Reviewed-by: Sarthak Sharma <sarthak.sharma@arm.com>
Acked-by: David Hildenbrand (Arm) <david@kernel.org>
Cc: Liam R. Howlett <liam@infradead.org>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Shuah Khan <shuah@kernel.org>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Vlastimil Babka <vbabka@kernel.org>

tools/testing/selftests/mm/.gitignore

@@ -2,7 +2,9 @@
 *
 !/**/
 !*.c
+*.mod.c
 !*.h
+local_config.h
 !*.sh
 !.gitignore
 !Makefile