compositor-wayland: Fix compiler warning

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>