Rename sub-BUILD file to the proper convention

Change-Id: Ib8c4044de06d2494ec579ef91ca7aa4eeb279512
diff --git a/WORKSPACE b/WORKSPACE
index 631ea78..a37e1d4 100644
--- a/WORKSPACE
+++ b/WORKSPACE
@@ -1,7 +1,7 @@
 new_local_repository(
   name = 'usr_repo',
   path = '/usr',
-  build_file = 'debian/BUILD.usr',
+  build_file = 'debian/usr.BUILD',
 )
 
 # TODO(brian): Make these point to something which isn't hard-coded to come off