Buck: Move //bucklets.defs import to .buckconfig

I767f582f8 added facility to import bucklets but included it in every
buck file.  This is what buildfile section in .buckconfig is for.

Change-Id: I2e7c34684779470514218da1a9332f6386934eda

.buckconfig

diff --git a/.buckconfig b/.buckconfig
index f143d0e..01c79be 100644
--- a/.buckconfig
+++ b/.buckconfig
@@ -5,6 +5,9 @@
   mvn_install = //:install
   mvn_deploy = //:deploy
 
+[buildfile]
+  includes = //bucklets.defs
+
 [java]
   src_roots = java, resources