toolchain/buildroot: add comment for glibc NaN-2008 headers requirement

toolchain/buildroot: add comment for glibc NaN-2008 headers requirement

Commit 4a5140ecf (toolchain/buildroot: glibc requires kernel headers >=
4.5 with NaN-2008) added a restriction on kernel headers for glibc when
the architecture is using naN-2008.

However, such a restriction is usually associated to a comment explaining
the restriction, so the user knows what is happening.

That comment was forgotten in 4a5140ecf. Add it now.

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