webkit: disable the libtool patch

The version of Webkit packaged in Buildroot uses libtool 2.x, for
which our package/buildroot-libtool.patch does not apply. So we
disable this patch. However, on my system, the lack of libtool
modification prevents the final linking to succeed (due to libtool
trying to link target code with host libraries). This will have to be
fixed separatly.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>