build: use -nostdinc and -isystem in NOSTDINC_FLAGS for out-of-tree kernel modules

This resolves issues uncovered by musl updates

Signed-off-by: Felix Fietkau <nbd@nbd.name>

include/kernel.mk

@@ -118,6 +118,9 @@ KERNEL_MAKE_FLAGS = \
 	cmd_syscalls= \
 	$(if $(__package_mk),KBUILD_EXTRA_SYMBOLS="$(wildcard $(PKG_SYMVERS_DIR)/*.symvers)")
 
+KERNEL_NOSTDINC_FLAGS = \
+	-nostdinc $(if $(DUMP),, -isystem $(shell $(TARGET_CC) -print-file-name=include))
+
 ifeq ($(call qstrip,$(CONFIG_EXTERNAL_KERNEL_TREE))$(call qstrip,$(CONFIG_KERNEL_GIT_CLONE_URI)),)
   KERNEL_MAKE_FLAGS += \
 	KERNELRELEASE=$(LINUX_VERSION)

package/kernel/ath10k-ct/Makefile

@@ -52,7 +52,8 @@ $(call KernelPackage/ath10k-ct)
   VARIANT:=smallbuffers
 endef
 
-NOSTDINC_FLAGS = \
+NOSTDINC_FLAGS := \
+	$(KERNEL_NOSTDINC_FLAGS) \
 	-I$(PKG_BUILD_DIR) \
 	-I$(STAGING_DIR)/usr/include/mac80211-backport/uapi \
 	-I$(STAGING_DIR)/usr/include/mac80211-backport \

package/kernel/mt76/Makefile

@@ -235,7 +235,8 @@ endef
 
 TARGET_CFLAGS += -I$(STAGING_DIR)/usr/include/libnl-tiny
 
-NOSTDINC_FLAGS = \
+NOSTDINC_FLAGS := \
+	$(KERNEL_NOSTDINC_FLAGS) \
 	-I$(PKG_BUILD_DIR) \
 	-I$(STAGING_DIR)/usr/include/mac80211-backport/uapi \
 	-I$(STAGING_DIR)/usr/include/mac80211-backport \

package/kernel/mwlwifi/Makefile

@@ -33,7 +33,8 @@ define KernelPackage/mwlwifi
   AUTOLOAD:=$(call AutoLoad,50,mwlwifi)
 endef
 
-NOSTDINC_FLAGS = \
+NOSTDINC_FLAGS := \
+	$(KERNEL_NOSTDINC_FLAGS) \
 	-I$(PKG_BUILD_DIR) \
 	-I$(STAGING_DIR)/usr/include/mac80211-backport/uapi \
 	-I$(STAGING_DIR)/usr/include/mac80211-backport \

package/kernel/rtl8812au-ct/Makefile

@@ -31,7 +31,8 @@ define KernelPackage/rtl8812au-ct
   PROVIDES:=kmod-rtl8812au
 endef
 
-NOSTDINC_FLAGS = \
+NOSTDINC_FLAGS := \
+	$(KERNEL_NOSTDINC_FLAGS) \
 	-I$(PKG_BUILD_DIR) \
 	-I$(PKG_BUILD_DIR)/include \
 	-I$(STAGING_DIR)/usr/include/mac80211-backport \