netsnmp: fix build with high BR2_JLEVEL settings

The netsnmp build system is not robust against parallel builds.

Fixes http://autobuild.buildroot.net/results/5bb827425ed07617a47a0c0d942a3c6c423fc02f

Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>

package/netsnmp/netsnmp.mk

@@ -21,6 +21,7 @@ NETSNMP_CONF_OPT = --with-persistent-directory=/var/lib/snmp --disable-static \
 		--with-mib-modules="host ucd-snmp/dlmod" \
 		--with-out-mib-modules="disman/event disman/schedule utilities" \
 		--with-out-transports="Unix"
+NETSNMP_MAKE = $(MAKE1)
 NETSNMP_BLOAT_MIBS = BRIDGE DISMAN-EVENT DISMAN-SCHEDULE DISMAN-SCRIPT EtherLike RFC-1215 RFC1155-SMI RFC1213 SCTP SMUX
 
 ifeq ($(BR2_ENDIAN),"BIG")