package/netsnmp: fix script net-snmp-create-v3-user's usage of 'ps'.

package/netsnmp: fix script net-snmp-create-v3-user's usage of 'ps'.

net-snmp-create-v3-user uses ps to check if snmpd is running.  To know
how to invoke 'ps', the build system use 'which ps' and does other
checks for the output format of 'ps', therefore inspecting 'ps' on the
build machine instead of the target.

If the build machine runs a OS like Debian, that uses a merged-usr and a
PATH of '/usr/bin:/bin', then 'which ps' returns /usr/bin/ps, which will
not work on the target if it does not also use a merged-usr.

Hardcode 'ps' to be /bin/ps to fix this issue and to improve build
reproducibility.

Signed-off-by: Nicolas Cavallari <nicolas.cavallari@green-communications.fr>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>