buildroot

package/strace: add missing libunwind optional dependency

This improve the reproducible build.

Signed-off-by: Romain Naour <romain.naour@gmail.com>
[Thomas: mention the libunwind dependency as being needed for 'strace
-k' in the Config.in help text, as suggested by Thomas De
Schampheleire.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>