package/glibc: set --enable-kernel to match kernel

Glibc is currently configured without any "--enable-kernel" option.
This causes it to use the oldest possible kernel API, slowing it down
and preventing it from using any kernel features from later versions.

Since we are likely building a kernel and matching glibc together,
backwards compatability is probably unnecessary so this patch
unconditionally configures glibc with --enable-kernel set to
BR2_TOOLCHAIN_HEADERS_AT_LEAST.

Signed-off-by: Sam Bobroff <sam.bobroff@au1.ibm.com>
Tested-by: Stewart Smith <stewart@linux.vnet.ibm.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>