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