collectd: add threshold.so, load.so and disk.so in PRIVATE_LIBS

to avoid the folloing warnings:
log.world.20140418_190059.log/bitbake.log:WARNING: gegl-0.2.0 was registered as shlib provider for load.so, changing it to collectd-5.2.2 because it was built later
log.world.20140418_190059.log/bitbake.log:WARNING:
libgphoto2-camlibs-2.4.11 was registered as shlib provider for disk.so, changing it to collectd-5.2.2 because it was built later
log.world.20140418_190059.log/bitbake.log:WARNING: gegl-0.2.0 was registered as shlib provider for threshold.so, changing it to
collectd-5.2.2 because it was built later

Signed-off-by: Jackie Huang <jackie.huang@windriver.com>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>