libtirpc: requires toolchain with threading support

Fixes:
http://autobuild.buildroot.org/results/8ba720f47f74df94b8c70ac4befd47c47ce65f2f



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