define CHENAB_SAI_SDK while building FBOSS for CHENAB sdk

Summary:
as titled

this is to indicate the build is using CHENAB_SAI_SDK implementation

Reviewed By: srikrishnagopu

Differential Revision: D66845304

fbshipit-source-id: 8ed546350f6c549b8e5ba1120ebb25913f3e1816

CMakeLists.txt

@@ -78,6 +78,7 @@ if ($ENV{CHENAB_SAI_SDK})
   set (CHENAB_SAI_SDK ON)
   link_directories(/var/FBOSS/sai_impl/lib)
   link_directories(/var/FBOSS/sdk/usr/lib64)
+  add_definitions (-DCHENAB_SAI_SDK)
 endif()
 
 option(BUILD_SAI_FAKE_LINK_TEST "Building of SAI Fake Link Test binaries" OFF)