kernel.mk: add 6.18 workaround for empty Module.symvers.tmp

On Linux 6.18 the path-based symvers filter can
yield an empty tmp file for external modules.

Add a 6.18-only fallback using modules.order
to populate Module.symvers.tmp.

Signed-off-by: Til Kaiser <[email protected]>
Link: https://github.com/openwrt/openwrt/pull/21078
Signed-off-by: Robert Marko <[email protected]>

include/kernel.mk

@@ -154,6 +154,9 @@ define collect_module_symvers
 		grep -F $(PKG_BUILD_DIR) $(PKG_BUILD_DIR)/$$$$subdir/Module.symvers >> $(PKG_BUILD_DIR)/Module.symvers.tmp; \
 		[ "$(PKG_BUILD_DIR)" = "$$$$realdir" ] || \
 			grep -F $$$$realdir $(PKG_BUILD_DIR)/$$$$subdir/Module.symvers >> $(PKG_BUILD_DIR)/Module.symvers.tmp; \
+		[ -s "$(PKG_BUILD_DIR)/Module.symvers.tmp" ] || [ "$(KERNEL_PATCHVER)" = "6.18" ] && \
+			sed 's/\.o$$$$//' $(PKG_BUILD_DIR)/$$$$subdir/modules.order | \
+			grep -Ff - $(PKG_BUILD_DIR)/$$$$subdir/Module.symvers >> $(PKG_BUILD_DIR)/Module.symvers.tmp; \
 	done; \
 	sort -u $(PKG_BUILD_DIR)/Module.symvers.tmp > $(PKG_BUILD_DIR)/Module.symvers; \
 	mkdir -p $(PKG_SYMVERS_DIR); \