posix: unistd.h: add declaration for _exit()
The `_exit()` function should be declared in `<unistd.h>`.
@@ -34,6 +34,8 @@ extern int unlink(const char *path);
extern int stat(const char *path, struct stat *buf);
extern int mkdir(const char *path, mode_t mode);
+FUNC_NORETURN void _exit(int status);
+
#ifdef CONFIG_NETWORKING
static inline int gethostname(char *buf, size_t len)
{
