fb-util: Include Executor.cpp

On systems with LD_BIND_NOW set or ghc with RTLD_DEFAULT linking enable,
compilaton of fb-util fails due to missing symbol
`common_hs_releaseGlobalCPUExecutor`. This is defined in Executor.cpp,
so we need to include it into the compilation.

Now I tried to add Utils/Executor.cpp and still got the errors. When
I moved the file to cpp/Executor.cpp and included it, it worked. Now I
have no clue why that is, and i tried to clean build artifacts but still
had issues when Executor.cpp is not in cpp/. So I left it, without
actually knowing _why_ this is.

common/util/fb-util.cabal

@@ -157,6 +157,7 @@ library
         cpp/HsStruct.cpp
         cpp/IOBuf.cpp
         cpp/EventBaseDataplane.cpp
+        cpp/Executor.cpp
         Util/AsanAlloc.cpp
         -- Util/GFlags.cpp