uclibc: fix clock_nanosleep declaration

Fixes autobuild errors:
 http://autobuild.buildroot.net/results/a58007263355e2734a8074d52b1b26b88973c39e
 http://autobuild.buildroot.net/results/7d6fd0872efd23c7de552ab637956902b43c3f58



Signed-off-by: Waldemar Brodkorb <wbx@openadk.org>
Reported-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>