Allow the flatbuffer compiler (flatc) to be built as a library.

Change-Id: I71baff427243f96be1596b01ff3405fdf39386d7
diff --git a/CMakeLists.txt b/CMakeLists.txt
index 6ac3996..a5d9817 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -48,6 +48,7 @@
   src/idl_gen_fbs.cpp
   src/idl_gen_grpc.cpp
   src/flatc.cpp
+  src/flatc_main.cpp
   grpc/src/compiler/schema_interface.h
   grpc/src/compiler/cpp_generator.h
   grpc/src/compiler/cpp_generator.cc