collectd: fix parallel build issues

For version 5.5.x a reorganization was made and some common funtionality
was moved to libraries, however proper dependency accounting isn't
handled in the Makefile thus causing build breakage on some parallel
builds. Fixes:
http://autobuild.buildroot.net/results/a5e/a5e6891e9ea66ac8216d3302da3702770ef7247b/



Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>