shadowsocks-libev: fix compiling with stale object files

shadowsocks-libev: fix compiling with stale object files

This will cause various kinds of failures in the buildbot

    ➜  shadowsocks-libev-3.0.7 git:(master) find . -name '*.o' -o -name '*.lo'
    ./libbloom/murmur2/libbloom_la-MurmurHash2.o
    ./libbloom/murmur2/libbloom_la-MurmurHash2.lo
    ./libbloom/libbloom_la-bloom.lo
    ./libbloom/libbloom_la-bloom.o
    ➜  shadowsocks-libev-3.0.7 git:(master) ls -l libbloom/murmur2
    total 24
    -rw-r--r-- 1 yousong yousong  324 Jun 27 14:17 libbloom_la-MurmurHash2.lo
    -rw-r--r-- 1 yousong yousong 4168 Jun 27 14:17 libbloom_la-MurmurHash2.o
    -rw-r--r-- 1 yousong yousong 1344 Feb 17 08:47 MurmurHash2.c
    -rw-r--r-- 1 yousong yousong  142 Feb 17 08:47 murmurhash2.h
    -rw-r--r-- 1 yousong yousong  213 Feb 17 08:47 README
    ➜  shadowsocks-libev-3.0.7 git:(master) file ./libbloom/murmur2/libbloom_la-MurmurHash2.o
    ./libbloom/murmur2/libbloom_la-MurmurHash2.o: ELF 64-bit LSB relocatable, x86-64, version 1 (SYSV), not stripped
    ➜  shadowsocks-libev-3.0.7 git:(master)

Signed-off-by: Yousong Zhou <yszhou4tech@gmail.com>