kernel: fix parallel build issues with header install

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

SVN-Revision: 39424

include/kernel-defaults.mk

@@ -104,7 +104,7 @@ define Kernel/Configure/Default
 	$(SCRIPT_DIR)/kconfig.pl 'm+' '+' $(LINUX_DIR)/.config.target /dev/null $(LINUX_DIR)/.config.override > $(LINUX_DIR)/.config
 	$(call Kernel/SetNoInitramfs)
 	rm -rf $(KERNEL_BUILD_DIR)/modules
-	[ -d $(LINUX_DIR)/user_headers ] || $(MAKE) $(KERNEL_MAKEOPTS) INSTALL_HDR_PATH=$(LINUX_DIR)/user_headers headers_install
+	$(_SINGLE) [ -d $(LINUX_DIR)/user_headers ] || $(MAKE) $(KERNEL_MAKEOPTS) INSTALL_HDR_PATH=$(LINUX_DIR)/user_headers headers_install
 	$(SH_FUNC) grep '=[ym]' $(LINUX_DIR)/.config | LC_ALL=C sort | md5s > $(LINUX_DIR)/.vermagic
 endef