package/glibc: add missing comment

Since [1], glibc package has a new option to install glibc utils to the
target. But this option is displayed below "Kernel Header Options".

As for uClibc package, add a comment line to separate Glibc options
from kernel header options.

[1] c6cd512f

Signed-off-by: Romain Naour <romain.naour@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>