package/freeradius-server: correctly set krb5-config

Fix typo added by commit 84d32e53:

configure: error: invalid variable name: `ac_cv_path_krb5-config'

Fixes:
 - http://autobuild.buildroot.org/results/5a0e5023c274223b5eb7623165395549e7f446bf



Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>