mdnsresponder: Also pass TARGET_CPPFLAGS

Use CFLAGS_DEBUG which is already used for TARGET_CFLAGS to also pass
TARGET_CPPFLAGS.

This fixes build issues with toolchains that don't automatically search
for header files (e.g: sys/capability.h), such as stbgcc-4.8-1.5

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

net/mdnsresponder/Makefile

@@ -86,7 +86,7 @@ endef
 # not found:
 MAKE_FLAGS += \
 	LD="$(TARGET_CC)"  \
-	CFLAGS_DEBUG="$(TARGET_CFLAGS)" \
+	CFLAGS_DEBUG="$(TARGET_CFLAGS) $(TARGET_CPPFLAGS)" \
 	LINKOPTS=-lc \
 	LDCONFIG= \
 	ETCBASE="$(PKG_INSTALL_DIR)/etc" \