Hi!

I was trying to build the library, but I got an error.

The problem was that the `gettid` function declaration from `/usr/include/bits/unistd_ext.h` is not static on my machine(s) (and that seems to be the default according to `man` and the sources of glibc and musl) but `gettid` is defined as static in `ucw/threads.c`.

Just removing the static keyword fixes the compilation error.

I just pushed a fix to master.

Have a nice fortnight