uninative: Add pthread linking workaround

uninative: Add pthread linking workaround

With newer glibc versions, pthread was merged into the main library so there is
no longer any need to link with -lpthread. On newer systems this means that linker
flag is dropped from places like pkgconfig .pc files. If uninative then uses this
newer library on an older system, the flag is missing and linker errors about missing
symbols occur.

Adding the linkage unconditionally to our uninative linking flags avoids that problem
at a potential cost of slight over linking.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>