adb: Also pass TARGET_CPPFLAGS

Fixes build issues with external toolchains that do not have STAGING_DIR
in their default search path.

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

package/utils/adb/Makefile

@@ -40,7 +40,7 @@ define Build/Compile
 	$(MAKE) -C $(PKG_BUILD_DIR)/adb/ \
 		$(TARGET_CONFIGURE_OPTS) \
 		TARGET=Linux \
-		CFLAGS="$(TARGET_CFLAGS)" \
+		CFLAGS="$(TARGET_CFLAGS) $(TARGET_CPPFLAGS)" \
 		LDFLAGS="$(TARGET_LDFLAGS)"
 endef