watchman: only enable eden support for FB internal builds

Summary:
This simplifies things for the OSS CI situation in
the short term, especially since Eden isn't quite buildable
or usable outside FB at this time.

Reviewed By: chadaustin

Differential Revision: D16953346

fbshipit-source-id: 028150bb3cfc9a8ccbe1197321214b56ad359463

build/fbcode_builder/manifests/watchman

@@ -25,7 +25,7 @@ fbcode/eden/fs = eden/fs
 [shipit.strip]
 ^fbcode/eden/fs/(?!.*\.thrift|service/shipit_test_file\.txt)
 
-[cmake.defines]
+[cmake.defines.fb=on]
 ENABLE_EDEN_SUPPORT=ON
 
 # FB macos specific settings