fix missing flowlib ppx during 'make js'
fix missing flowlib ppx during 'make js'

Summary:
`make clean; make js` fails because the ppx for flowlib hasn't been
built. we don't notice it in CI because we `make` the native binary first, which
does depend on the ppx.

Reviewed By: gabelevi

Differential Revision: D5510463

fbshipit-source-id: b0ac319a1f3d7a800f82b2acc85a99c92d48f3d9

ylu1317/flow,ylu1317/flow,samwgoldman/flow,nmote/flow,facebook/flow,JonathanUsername/flow,gabelevi/flow,JonathanUsername/flow,samwgoldman/flow,popham/flow,gabelevi/flow,gabelevi/flow,popham/flow,JonathanUsername/flow,TiddoLangerak/flow,samwgoldman/flow,nmote/flow,jamesgpearce/flow,gabelevi/flow,facebook/flow,mroch/flow,mroch/flow,jamesgpearce/flow,samwgoldman/flow,MichaelDeBoey/flow,mroch/flow,facebook/flow,facebook/flow,JonathanUsername/flow,jamesgpearce/flow,TiddoLangerak/flow,TiddoLangerak/flow,claudiopro/flow,mroch/flow,jamesgpearce/flow,facebook/flow,ylu1317/flow,JonathanUsername/flow,samwgoldman/flow,MichaelDeBoey/flow,samwgoldman/flow,MichaelDeBoey/flow,popham/flow,AgentME/flow,nmote/flow,gabelevi/flow,samwgoldman/flow,JonathanUsername/flow,nmote/flow,jamesgpearce/flow,AgentME/flow,TiddoLangerak/flow,AgentME/flow,claudiopro/flow,MichaelDeBoey/flow,facebook/flow,jamesgpearce/flow,popham/flow,MichaelDeBoey/flow,jamesgpearce/flow,ylu1317/flow,nmote/flow,ylu1317/flow,mroch/flow,JonathanUsername/flow,nmote/flow,popham/flow,mroch/flow,claudiopro/flow,nmote/flow,popham/flow,claudiopro/flow,mroch/flow,facebook/flow,popham/flow,claudiopro/flow,gabelevi/flow,AgentME/flow,gabelevi/flow,TiddoLangerak/flow,AgentME/flow