package/c-libraries: need linux-headers

Now that we check that a target package in the _DEPENDENCIES of another
package has to be enabled in config, all target packages must have a
kconfig symbol.

Add a Kconfig symbol for linux-headers, and select it from the packages
that depends on it (C libraries).

Also remove the now-misleading comments "for legal-info" from the C
libraries.

Fixes:
    http://autobuild.buildroot.org/results/2a9/2a9e5d27b34357819b44f573a834da1ba5079030/


    ... and numerous similar failures ...

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>