snmppp: update download URL

The filename of the SNMP++ sources has changed.
The old one is still working because the server automatically redirects to the
new file, but it is safer to use the official name.

Signed-off-by: Luca Ceresoli <luca@lucaceresoli.net>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/snmppp/snmppp.mk

@@ -5,7 +5,7 @@
 ################################################################################
 
 SNMPPP_VERSION = 3.3.4
-SNMPPP_SOURCE = snmp++$(SNMPPP_VERSION).tar.gz
+SNMPPP_SOURCE = snmp++-$(SNMPPP_VERSION).tar.gz
 SNMPPP_SITE = http://www.agentpp.com
 SNMPPP_DEPENDENCIES = openssl host-pkgconf
 SNMPPP_INSTALL_STAGING = YES