reaver: Fix build with external toolchains

We would not be able to search for pcap.h because CFLAGS are not passed
from src/Makefile down to src/wps/Makefile:

make[4]: Entering directory
'/home/florian/dev/openwrt/trunk/build_dir/target-mipsel-unknown-linux-gnu_glibc/reaver-1.4/src/wps'
mipsel-linux-gnu-gcc -I../utils -I ../ wps_attr_build.c -c
mipsel-linux-gnu-gcc -I../utils -I ../ wps_attr_parse.c -c
mipsel-linux-gnu-gcc -I../utils -I ../ wps_attr_process.c -c
mipsel-linux-gnu-gcc -I../utils -I ../ wps.c -c
mipsel-linux-gnu-gcc -I../utils -I ../ wps_common.c -c
mipsel-linux-gnu-gcc -I../utils -I ../ wps_dev_attr.c -c
mipsel-linux-gnu-gcc -I../utils -I ../ wps_enrollee.c -c
mipsel-linux-gnu-gcc -I../utils -I ../ wps_registrar.c -c
In file included from ../misc.h:41:0,
                 from wps_registrar.c:27:
../defs.h:43:18: fatal error: pcap.h: No such file or directory
 #include <pcap.h>
                  ^
compilation terminated.
Makefile:28: recipe for target 'wps_registrar.o' failed

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>

net/reaver/Makefile

@@ -9,7 +9,7 @@ include $(TOPDIR)/rules.mk
 
 PKG_NAME:=reaver
 PKG_VERSION:=1.4
-PKG_RELEASE:=2
+PKG_RELEASE:=3
 PKG_MAINTAINER:=Yousong Zhou <yszhou4tech@gmail.com>
 
 PKG_SOURCE:=$(PKG_NAME)-$(PKG_VERSION).tar.gz
@@ -25,6 +25,7 @@ include $(INCLUDE_DIR)/package.mk
 
 CONFIGURE_PATH:=src
 MAKE_PATH:=src
+EXTRA_CFLAGS=$(TARGET_CPPFLAGS)
 
 define Package/reaver
   SECTION:=net