h2_leveldb_impl_drv.cc doesn't compile under Erlang/OTP 17.5 or newer due to missing field 'emergency_close'

[tatsuya6502]

c_src/h2leveldb_impl_drv.cc:235:1: 
error: missing field 'emergency_close' initializer [-Werror,-Wmissing-field-initializers]

The emergency_close field was introduced to Erlang/OTP 17.5 by this commit:
erlang/otp@d373dc4

erts: Extend driver interface with emergency_close
The intention of this callback is to close all sockets associated to a port. It is closed only on crashdumps.

This will currently only be used for the epmd port.

[tatsuya6502]

Just for now, I simply added the new field emergency_close to ErlDrvEntry so that it does compile under Erlang/OTP 17.5. However, it doesn't compile under R16 anymore. I'll revisit this later to make it work under both 17.x and R16.

Commit: 0c1af44

Tested on Erlang 17.5. These simple eunit cases passed.

% freebsd-version 
10.1-RELEASE-p10
% . ~/erlang/17.5_hipe/activate
% gmake clean eunit
...
==> h2leveldb (eunit)
  All 8 tests passed.