cairo: fix build when toolchain has no thread support

Fixes the build failure visible at
http://autobuild.buildroot.net/results/d49/d491de5a774592929a59f3bfd0b1fd8cbc089ac0



Signed-off-by: Samuel Martin <s.martin49@gmail.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>