package/ipmitool: fix per-package build

Fix the following per-package build failure raised since commit
82480ee2:

configure.ac:44: error: possibly undefined macro: AC_SEARCH_LIBS
      If this token and others are legitimate, please use m4_pattern_allow.
      See the Autoconf documentation.

Fixes:
 - http://autobuild.buildroot.org/results/ecf72cd1ebd53f3b3b10f0a24082f9c0ab5d4d12



Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>