libweston: Properly namespace backends entrypoint

This prevents loading a backend as a simple module. This will avoid
messing up with backends when we will introduce libweston common
modules.

Signed-off-by: Quentin Glidic <sardemff7+git@sardemff7.net>
Reviewed-by: Daniel Stone <daniels@collabora.com>
diff --git a/libweston/compositor-drm.c b/libweston/compositor-drm.c
index c7f7f63..8116052 100644
--- a/libweston/compositor-drm.c
+++ b/libweston/compositor-drm.c
@@ -3304,8 +3304,8 @@
 }
 
 WL_EXPORT int
-backend_init(struct weston_compositor *compositor,
-	     struct weston_backend_config *config_base)
+weston_backend_init(struct weston_compositor *compositor,
+		    struct weston_backend_config *config_base)
 {
 	struct drm_backend *b;
 	struct weston_drm_backend_config config = {{ 0, }};