Enabled rebuild of flatc when it already exists.
Enabled rebuild of flatc when it already exists.

Tested:
Touched src/flatc.cpp in the flatbuffers project and verified bin/flatc
is rebuilt when building the Android project.

Bug: 23345877
Change-Id: Ibd894d03f7a60c51397de3f857d34bc348f24e8e

google/zooshi,google/zooshi,google/zooshi,google/zooshi,google/zooshi