compositor-wayland: Fix compiler warning

sizeof returns size_t, for which the correct printf specifier is %zu.
Fixes the following warning when building for ARMv7.

	src/compositor-wayland.c: In function 'wayland_output_get_shm_buffer':
	src/compositor-wayland.c:260:3: warning: format '%ld' expects argument of type 'long int', but argument 2 has type 'unsigned int' [-Wformat=]
	   weston_log("could not zalloc %ld memory for sb: %m\n", sizeof *sb);
	   ^

Signed-off-by: Thierry Reding <treding@nvidia.com>

src/compositor-wayland.c

@@ -257,7 +257,7 @@ wayland_output_get_shm_buffer(struct wayland_output *output)
 
 	sb = zalloc(sizeof *sb);
 	if (sb == NULL) {
-		weston_log("could not zalloc %ld memory for sb: %m\n", sizeof *sb);
+		weston_log("could not zalloc %zu memory for sb: %m\n", sizeof *sb);
 		close(fd);
 		free(data);
 		return NULL;