gettid is not static

Hello, 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. Prokop