package/s390-tools: fix build with netsnmp

package/s390-tools: fix build with netsnmp

Fix the following build failure:

/bin/sh: net-snmp-config: command not found
/home/buildroot/autobuild/run/instance-2/output-1/host/lib/gcc/s390x-buildroot-linux-gnu/9.3.0/../../../../s390x-buildroot-linux-gnu/bin/ld: osasnmpd.o: in function `main':
osasnmpd.c:(.text.startup+0xcc): undefined reference to `snmp_log_perror'

Moreover, replace perl-net-snmp dependency by netsnmp as osasnmpd is an
SNMP subagent for the net-snmp package:
https://github.com/ibm-s390-tools/s390-tools/blob/master/osasnmpd/osasnmpd.8

Fixes:
 - http://autobuild.buildroot.org/results/00796f2ebd5fb0e08ac7a05a9ee566f2bc4bd1c3

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Reviewed-by: Alexander Egorenkov <egorenar@linux.ibm.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>