test_client.c: error: static declaration of 'yield' follows non-static declaration on Solaris

Solaris 11.4 and OpenIndiana define yield(2) in unistd.h. Defining function of the same name in test_client.c fails with:

test_client.c:329:1: error: static declaration of 'yield' follows non-static declaration
  329 | yield(void) {
      | ^~~~~
In file included from test_client.c:24:
/usr/include/unistd.h:563:13: note: previous declaration of 'yield' was here
  563 | extern void yield(void);
      |             ^~~~~

/usr/include/unistd.h:

#if !defined(__XOPEN_OR_POSIX) || defined(__EXTENSIONS__)
extern void yield(void);
#endif /* !defined(__XOPEN_OR_POSIX) || defined(__EXTENSIONS__) */