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>

net/shadowsocks-libev/Makefile

@@ -19,7 +19,7 @@ include $(TOPDIR)/rules.mk
 #
 PKG_NAME:=shadowsocks-libev
 PKG_VERSION:=3.0.7
-PKG_RELEASE:=2
+PKG_RELEASE:=3
 
 PKG_SOURCE:=$(PKG_NAME)-$(PKG_VERSION).tar.gz
 PKG_SOURCE_URL:=https://github.com/shadowsocks/shadowsocks-libev/releases/download/v$(PKG_VERSION)
@@ -107,6 +107,16 @@ uci batch <<-EOF
 EOF
 endef
 
+define Build/Prepare
+	$(call Build/Prepare/Default)
+	$(FIND) $(PKG_BUILD_DIR) \
+			   -name '*.o' \
+			-o -name '*.lo' \
+			-o -name '.deps' \
+			-o -name '.libs' \
+		| xargs rm -rvf
+endef
+
 CONFIGURE_ARGS += \
 	--disable-documentation \
 	--disable-silent-rules \