write_riemann: remove unneeded include
Fixes build on NetBSD:
Making all in daemon
CC write_riemann_threshold.lo
write_riemann_threshold.c:36:18: fatal error: ltdl.h: No such file or directory
#include <ltdl.h>
^
compilation terminated.
*** Error code 1
Stop.
make[3]: stopped in /home/ruben/src/collectd/src
*** Error code 1
Stop.
make[2]: stopped in /home/ruben/src/collectd/src
*** Error code 1
Stop.
make[1]: stopped in /home/ruben/src/collectd/src
*** Error code 1
Stop.
make: stopped in /home/ruben/src/collectd