identity: Add id_drm.c to SCons build.

This was missed in commit f7cbaae13d.
(cherry picked from commit 02ee7c2950)

src/gallium/drivers/identity/SConscript

@@ -5,9 +5,10 @@ env = env.Clone()
 identity = env.ConvenienceLibrary(
 	target = 'identity',
 	source = [
-		'id_screen.c',
 		'id_context.c',
+		'id_drm.c',
 		'id_objects.c',
+		'id_screen.c',
 	])
 
 Export('identity')