rabbitmq-c: needs a toolchain with thread support

Fixes:
http://autobuild.buildroot.net/results/312/312f0e59e3f83868e86725ee552b320d0e739f9f/



[Peter: add autobuilder reference]
Signed-off-by: Joris Lijssens <joris.lijssens@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>