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.c b/libweston/compositor.c
index a5ce76d..2660264 100644
--- a/libweston/compositor.c
+++ b/libweston/compositor.c
@@ -5399,7 +5399,7 @@
 	if (backend >= ARRAY_LENGTH(backend_map))
 		return -1;
 
-	backend_init = weston_load_module(backend_map[backend], "backend_init");
+	backend_init = weston_load_module(backend_map[backend], "weston_backend_init");
 	if (!backend_init)
 		return -1;