build: skip kernel stack validation when building on macOS

Since we switched to 4.19, the kernel build checks for libelf to decide if
it should build tools for stack validation.

On macOS, this check fails during target/compile, but succeeds during package
build (because of the pkg-config path picking up target libraries).

Signed-off-by: Felix Fietkau <nbd@nbd.name>
(cherry picked from commit c3e31b6a9b)

include/kernel.mk

@@ -129,6 +129,10 @@ ifdef CONFIG_USE_SPARSE
   KERNEL_MAKEOPTS += C=1 CHECK=$(STAGING_DIR_HOST)/bin/sparse
 endif
 
+ifeq ($(HOST_OS),Darwin)
+  export SKIP_STACK_VALIDATION:=1
+endif
+
 PKG_EXTMOD_SUBDIRS ?= .
 
 define populate_module_symvers