buildhistory.bbclass: fix shell syntax when using dash

* fix:
  run.buildhistory_get_image_installed.791888: 198: [: unexpected operator

  introduced in:
  commit 82e6172c
  Author: Andres Beltran <abeltran@linux.microsoft.com>
  Date:   Tue Oct 5 00:34:15 2021 +0000

    buildhistory: Fix package output files for SDKs

Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>
Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>