iproute2: Fix KERNEL_INCLUDE in SDK

iproute2: Fix KERNEL_INCLUDE in SDK

In the SDK the folder $(LINUX_DIR)/user_headers/include does not exist,
but it more or less contains the same content as
$(LINUX_DIR)/include/uapi which also exists in the SDK.

Since iproute2 commit 1d819dcc741e ("configure: fix parsing issue on
include_dir option") it checks if this folder exists and aborts the
build if it does not exists.
https://git.kernel.org/pub/scm/network/iproute2/iproute2.git/commit/?id=1d819dcc741e25958190e31f8186c940713fa0a8

With this commit the KERNEL_INCLUDE variable points to a valid folder
with the kernel include headers. I am not sure if they are actually
needed because the build worked before even with an invalid path.

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>