libsmi: set the SH to the target path

Set the SH to the target path to avoid host contamination,
which causes the following issue on some hosts:
| WARNING: QA Issue: /usr/bin/smicache_libsmi contained in
  package libsmi requires /usr/bin/sh, but no providers
  found in its RDEPENDS [file-rdeps]
| Computing transaction...error: Can't install
  libsmi-0.4.8-r0.0@corei7_64: no package provides /usr/bin/sh

Signed-off-by: Jackie Huang <jackie.huang@windriver.com>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>