package/sg3_utils: ensure to build against librt when needed

The sg3_utils has provisions to build against librt when needed, but
forgot to use that mechanism for the sg_turs program. This commit
fixes that. The patch has been submitted upstream to the sg3_utils
author.

Fixes:

  http://autobuild.buildroot.net/results/67b890a41d05497820ea4f44e187257dd6818b0b/



Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Tested-by: Matthew Weber <matthew.weber@rockwellcollins.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>