Add unsupported_poll

Sorry, missing unsupported_poll broke compilation after the previous patch

fs/unsupported.c

@@ -63,3 +63,8 @@ int unsupported_write(struct file *fp, struct uio *uio, int flags)
 {
     return EINVAL; /* as explained above, doesn't matter what we return */
 }
+
+int unsupported_poll(struct file *fp, int events)
+{
+    return 0;
+}

fs/unsupported.h

@@ -13,6 +13,7 @@ fo_truncate_t unsupported_truncate;
 fo_ioctl_t unsupported_ioctl;
 fo_stat_t unsupported_stat;
 fo_chmod_t unsupported_chmod;
+fo_poll_t unsupported_poll;
 
 __END_DECLS