Use FFI::Library::CURRENT_PROCESS for Socket

* That way it's more consistent with what Truffle::POSIX does,
  and avoids a separate library handle.

Author: eregon

lib/truffle/socket/truffle/foreign.rb

@@ -28,7 +28,7 @@ module Truffle
   module Socket
     module Foreign
       extend ::FFI::Library
-      ffi_lib ::FFI::Library::LIBC
+      ffi_lib ::FFI::Library::CURRENT_PROCESS
 
       SIZEOF_INT = ::FFI.type_size(:int)