buffer: make enum wlr_buffer_cap public

Custom backends and renderers need to implement
wlr_backend_impl.get_buffer_caps and
wlr_renderer_impl.get_render_buffer_caps. They can't if enum
wlr_buffer_cap isn't made public.

(cherry picked from commit a38baec1f8)

render/gles2/renderer.c

@@ -17,7 +17,6 @@
 #include <wlr/util/log.h>
 #include "render/gles2.h"
 #include "render/pixel_format.h"
-#include "types/wlr_buffer.h"
 
 static const GLfloat verts[] = {
 	1, 0, // top right