security/gnutls: initialise libpthread

To ensure thread-safety libgnutls calls libpthread functions but to
avoid the overhead for single-threaded programs it does not link with
libpthread. It only calls libpthread if the executable or another
library links to it.

Since 3.8.0 libgnutls calls pthread_key_create from its init function
but because it does not link with libpthread libpthread might not have
been initialised yet. Patch the libgnutls init function so it
initialises libpthread.

PR: 278076