[source] rules.mk: add missing CPP definition

[LEDE Commits]

blogic pushed a commit to source.git, branch master:
https://git.lede-project.org/569f74ef49bae9326a723050a400346686983d86

commit 569f74ef49bae9326a723050a400346686983d86
Author: BangLang Huang <banglang.huang at foxmail.com>
AuthorDate: Mon Aug 14 12:49:10 2017 +0800

    rules.mk: add missing CPP definition
    
    Signed-off-by: BangLang Huang <banglang.huang at foxmail.com>
---
 rules.mk | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/rules.mk b/rules.mk
index ed796a5..43d8efe 100644
--- a/rules.mk
+++ b/rules.mk
@@ -260,6 +260,7 @@ endif
 BUILD_KEY=$(TOPDIR)/key-build
 
 TARGET_CC:=$(TARGET_CROSS)gcc
+TARGET_CPP:=$(TARGET_CROSS)cpp
 TARGET_CXX:=$(TARGET_CROSS)g++
 KPATCH:=$(SCRIPT_DIR)/patch-kernel.sh
 SED:=$(STAGING_DIR_HOST)/bin/sed -i -e
@@ -300,6 +301,7 @@ TARGET_CONFIGURE_OPTS = \
   NM="$(TARGET_NM)" \
   CC="$(TARGET_CC)" \
   GCC="$(TARGET_CC)" \
+  CPP="$(TARGET_CPP)" \
   CXX="$(TARGET_CXX)" \
   RANLIB="$(TARGET_RANLIB)" \
   STRIP=$(TARGET_CROSS)strip \