posix: ioctl: actually declare a function prototype
The `<sys/ioctl.h>` header never actually declared
the `ioctl(2)` function prototype, so this fixes that.
#define FIONBIO 0x5421
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+int ioctl(int fd, unsigned long request, ...);
+
+#ifdef __cplusplus
+}
+#endif
+
#endif /* ZEPHYR_INCLUDE_POSIX_SYS_IOCTL_H_ */
