collectd: fix option passed to the snmp dependency

--with-libnetsnmp doesn't take a path to the netsnmp config script,
but to the sysroot where netsnmp can be found.

Fixes:

  http://autobuild.buildroot.org/results/358e2f02f42e08fe362ecd46e636c0048e77c21f/



Signed-off-by: Jack Kao <jackzzjack@gmail.com>
[Thomas: path is $(STAGING_DIR)/usr, not $(STAGING_DIR)/usr/include.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>