build: fix kernel headers install for uml

The kernel headers makefile needs to override LINUX_KARCH

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

include/kernel.mk

@@ -97,7 +97,7 @@ endif
 
 KERNEL_MAKE = $(MAKE) $(KERNEL_MAKEOPTS)
 
-KERNEL_MAKE_FLAGS := \
+KERNEL_MAKE_FLAGS = \
 	HOSTCFLAGS="$(HOST_CFLAGS) -Wall -Wmissing-prototypes -Wstrict-prototypes" \
 	CROSS_COMPILE="$(KERNEL_CROSS)" \
 	ARCH="$(LINUX_KARCH)" \