[openwrt/openwrt] bpf-headers: fix build error from within the SDK

[LEDE Commits]

nbd pushed a commit to openwrt/openwrt.git, branch master:
https://git.openwrt.org/8c1d5129ee8464b3a240d73c48eb2d166537c1a8

commit 8c1d5129ee8464b3a240d73c48eb2d166537c1a8
Author: Felix Fietkau <nbd at nbd.name>
AuthorDate: Fri Feb 4 12:48:45 2022 +0100

    bpf-headers: fix build error from within the SDK
    
    The SDK does not ship the generic platform files. Use relative path for
    GENERIC_PLATFORM_DIR to make it work. This points it at the files from
    the feed directory instead of the base SDK path
    
    Signed-off-by: Felix Fietkau <nbd at nbd.name>
---
 package/kernel/bpf-headers/Makefile | 1 +
 1 file changed, 1 insertion(+)

diff --git a/package/kernel/bpf-headers/Makefile b/package/kernel/bpf-headers/Makefile
index f3c0584007..0f0857133a 100644
--- a/package/kernel/bpf-headers/Makefile
+++ b/package/kernel/bpf-headers/Makefile
@@ -20,6 +20,7 @@ PKG_SOURCE_URL:=$(LINUX_SITE)
 PKG_HASH:=$(LINUX_KERNEL_HASH-$(strip $(PKG_VERSION)))
 PKG_BUILD_DIR:=$(KERNEL_BUILD_DIR)/bpf-headers/$(PKG_NAME)-$(PKG_VERSION)
 
+GENERIC_PLATFORM_DIR := $(CURDIR)/../../../target/linux/generic
 GENERIC_BACKPORT_DIR := $(GENERIC_PLATFORM_DIR)/backport$(if $(wildcard $(GENERIC_PLATFORM_DIR)/backport-$(PKG_PATCHVER)),-$(PKG_PATCHVER))
 GENERIC_PATCH_DIR := $(GENERIC_PLATFORM_DIR)/pending$(if $(wildcard $(GENERIC_PLATFORM_DIR)/pending-$(PKG_PATCHVER)),-$(PKG_PATCHVER))
 GENERIC_HACK_DIR := $(GENERIC_PLATFORM_DIR)/hack$(if $(wildcard $(GENERIC_PLATFORM_DIR)/hack-$(PKG_PATCHVER)),-$(PKG_PATCHVER))