blktrace: add missing dependency on Config.in comment

The BR2_PACKAGE_BLKTRACE option "depends on
BR2_PACKAGE_LIBAIO_ARCH_SUPPORTS", but this architecture dependency was
not replicated in the Config.in comment. This commit fixes this
inconsistency.

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