smartsnmpd: Properly pass down TARGET_CPPFLAGS

The smartsnmpd SConstruct file only accepts setting CFLAGS and does not
use CPPFLAGS, so pass both down using CFLAGS.

This fixes build errors with external toolchains that don't
automatically search for headers in $(STAGING_DIR).

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

net/smartsnmpd/Makefile

@@ -47,6 +47,7 @@ SCONS_OPTIONS += --transport=uloop
 define Build/Configure
 	(cd $(PKG_BUILD_DIR); \
 		$(SCONS_VARS) \
+		CFLAGS="$(TARGET_CFLAGS) $(TARGET_CPPFLAGS)" \
 		scons \
 			prefix=/usr \
 			$(SCONS_OPTIONS) \