fbterm: fix linking issue when static linking and a toolchain with threads

Fixes:
http://autobuild.buildroot.net/results/899/899eb7e36dc31b7dcb51c0dd23d91965111d22c4/
http://autobuild.buildroot.net/results/43a/43a34d8b64f536caa579709508bb5e00633ba6e7/
http://autobuild.buildroot.net/results/f34/f343d015b127f29d614e7ea38b173d3a5c270d84/
http://autobuild.buildroot.net/results/da8/da88f40dfc51aea122c146e5bfee19b031cfaab6/



And many more.

Fontconfig uses pthreads if available, but fbterm forgets to link with it
breaking static builds.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>