gecko: unbreak with libevent >= 2.1

In file included from objdir//ipc/chromium/Unified_cpp_ipc_chromium0.cpp:119:
ipc/chromium/src/base/message_pump_libevent.cc:29:1: error: use of undeclared identifier '_EVENT_SIZEOF_LONG'
CHECK_EVENT_SIZEOF(LONG, long);
^
ipc/chromium/src/base/message_pump_libevent.cc:26:19: note: expanded from macro 'CHECK_EVENT_SIZEOF'

static_assert(_EVENT_SIZEOF_##TYPE == sizeof(type), \
              ^

<scratch space>:50:1: note: expanded from here
_EVENT_SIZEOF_LONG
^

PR: 216527
Obtained from: upstream