dnsmasq: Pass TARGET_CPPFLAGS to Makefile

With the introduction of the ubus notifications, we would now fail building
dnsmasq with external toolchains that don't automatically search for headers.
Pass TARGET_CPPFLAGS to the Makefile to resolve that.

Fixes: 34a206bc11 ("dnsmasq: add ubus notifications for new leases")
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>

package/network/services/dnsmasq/Makefile

@@ -138,7 +138,7 @@ endif
 
 MAKE_FLAGS := \
 	$(TARGET_CONFIGURE_OPTS) \
-	CFLAGS="$(TARGET_CFLAGS)" \
+	CFLAGS="$(TARGET_CFLAGS) $(TARGET_CPPFLAGS)" \
 	LDFLAGS="$(TARGET_LDFLAGS)" \
 	COPTS="$(COPTS)" \
 	PREFIX="/usr"