collectd: fix incorrect select of libmodbus

BR2_PACKAGE_MODBUS does not exist, we meant to select
BR2_PACKAGE_LIBMODBUS.

Fixes:

  http://autobuild.buildroot.org/results/661/6617ea58424bf55640a70bee8fc3a6a8d098e3b2/



Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>