linux-sync-file.c uses Linux sync_file ioctls and is only needed for the
DRM backend; compile it conditionally.

Index: libweston/meson.build
--- libweston/meson.build.orig
+++ libweston/meson.build
@@ -33,7 +33,7 @@ srcs_libweston = [
 	'input.c',
 	'linux-dmabuf.c',
 	'linux-explicit-synchronization.c',
-	'linux-sync-file.c',
+	get_option('backend-drm') ? 'linux-sync-file.c' : [],
 	'log.c',
 	'noop-renderer.c',
 	'output-capture.c',
