build: Make the X11 compositor explicitely depend on xcb-shm

Otherwise, it means the X11 compositor depends on another library to
pull xcb-shm (cairo?), which is not always the case. Here I end up with:

[01:54:38.970] Failed to load module:
$prefix/lib/weston/x11-backend.so: undefined symbol: xcb_shm_id

configure.ac

@@ -93,7 +93,7 @@ if test x$enable_x11_compositor = xyes; then
   LIBS=$xcb_save_LIBS
   CFLAGS=$xcb_save_CFLAGS
 
-  X11_COMPOSITOR_MODULES="x11 x11-xcb"
+  X11_COMPOSITOR_MODULES="x11 x11-xcb xcb-shm"
 
   PKG_CHECK_MODULES(X11_COMPOSITOR_XKB, [xcb-xkb],
 		    [have_xcb_xkb="yes"], [have_xcb_xkb="no"])