ppp: fix build with kernel 4.14.9+

With a9772285a724 ("linux/compiler.h: Split into compiler.h and
compiler_types.h") compiler.h was refactored and most its content was
moved to compiler_types.h. Both files are required to build ppp-mod-pppoa.

Signed-off-by: Mathias Kresin <dev@kresin.me>

package/network/services/ppp/Makefile

@@ -178,10 +178,15 @@ $(call Build/Configure/Default,, \
 	UNAME_M="$(ARCH)" \
 )
 	mkdir -p $(PKG_BUILD_DIR)/pppd/plugins/pppoatm/linux
-	cp \
+	$(CP) \
 		$(LINUX_DIR)/include/linux/compiler.h \
 		$(LINUX_DIR)/include/$(LINUX_UAPI_DIR)linux/atm*.h \
 		$(PKG_BUILD_DIR)/pppd/plugins/pppoatm/linux/
+
+	# Kernel 4.14.9+ only, ignore the exit status of cp in case the file
+	# doesn't exits
+	-$(CP) $(LINUX_DIR)/include/linux/compiler_types.h \
+		$(PKG_BUILD_DIR)/pppd/plugins/pppoatm/linux/
 endef
 
 TARGET_CFLAGS += -ffunction-sections -fdata-sections