uclibc: fix MIPS builds with disabled threads

There was a regression found while building for MIPS
with disabled threads. Upstream patch fixed it.

Signed-off-by: Waldemar Brodkorb <wbx@openadk.org>
Acked-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Reviewed-by: Vicente Olivert Riera <Vincent.Riera@imgtec.com>
Tested-by: Vicente Olivert Riera <Vincent.Riera@imgtec.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>