rsocket: fixup .gitinore

Summary:
the `build` entry is preventing shipit from keeping that
directory in sync

Reviewed By: phoad

Differential Revision: D14643445

fbshipit-source-id: 413156786d479b41a6765f3ae1390d52845adf90

Author: wez

Diff for: .gitignore

@@ -46,7 +46,6 @@ install_manifest.txt
 
 ### Project ###
 
-/build
 /reactivesocket-cpp/CTestTestfile.cmake
 /reactivesocket-cpp/ReactiveSocketTest
 /reactivesocket-cpp/compile_commands.json