tests: make create_shm_buffer() static

No users remain outside the file. This will allow to fix the assumptions
in the function.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Reviewed-by: Daniel Stone <daniels@collabora.com>
diff --git a/tests/weston-test-client-helper.c b/tests/weston-test-client-helper.c
index 4f610f7..bfe9a0b 100644
--- a/tests/weston-test-client-helper.c
+++ b/tests/weston-test-client-helper.c
@@ -414,7 +414,7 @@
 	surface_leave
 };
 
-struct wl_buffer *
+static struct wl_buffer *
 create_shm_buffer(struct client *client, int width, int height, void **pixels)
 {
 	struct wl_shm *shm = client->wl_shm;
diff --git a/tests/weston-test-client-helper.h b/tests/weston-test-client-helper.h
index 6f2d05b..3e1cfcd 100644
--- a/tests/weston-test-client-helper.h
+++ b/tests/weston-test-client-helper.h
@@ -157,9 +157,6 @@
 struct client *
 create_client_and_test_surface(int x, int y, int width, int height);
 
-struct wl_buffer *
-create_shm_buffer(struct client *client, int width, int height, void **pixels);
-
 struct buffer *
 create_shm_buffer_a8r8g8b8(struct client *client, int width, int height);