klibc_2.0.2: fix hardcoded symlinks to linux-libc-headers

Klibc needs a copy of the linux headers for itself to build.
On target the libs and a second copy of headers are installed as well:
we link relatively to the staging sysroot of the target to make
these headers available when necessary.

Signed-off-by: Andrea Adami <andrea.adami@gmail.com>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>