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>

tests/weston-test-client-helper.c

@@ -414,7 +414,7 @@ static const struct wl_surface_listener surface_listener = {
 	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;

tests/weston-test-client-helper.h

@@ -157,9 +157,6 @@ create_client(void);
 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);