package/libv4l: disable for musl toolchains

musl toolchains are not supported yet in libv4l but
it's a work in progress.

see:
http://git.linuxtv.org/cgit.cgi/v4l-utils.git/commit/?id=70570c6095fa5859cecd0f9522fe7fd749f1d554
http://git.linuxtv.org/cgit.cgi/v4l-utils.git/commit/?id=403a4e2697a1ff96fe2fa16589039595f21cadf0



But it's not enough.

Disable musl toolchains for libv4l 1.6.3 and wait for a new release.

[Thomas: update Config.in comment to take into account the uclibc or
glibc dependency.]

Signed-off-by: Romain Naour <romain.naour@openwide.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>