Remove redundant declaration of v4l2_ioctl

There is no need to declare mpers printers in defs.h because
they are already declared in printers.h or native_printer_decls.h.

This complements commit 1e56814f77594a22a74ddee629639eb0bb7edf3a.

* defs.h (v4l2_ioctl): Remove.
diff --git a/defs.h b/defs.h
index 41e3c43..f693da1 100644
--- a/defs.h
+++ b/defs.h
@@ -670,7 +670,6 @@
 extern int term_ioctl(struct tcb *, const unsigned int, long);
 extern int ubi_ioctl(struct tcb *, const unsigned int, long);
 extern int uffdio_ioctl(struct tcb *, const unsigned int, long);
-extern int v4l2_ioctl(struct tcb *, const unsigned int, long);
 
 extern int tv_nz(const struct timeval *);
 extern int tv_cmp(const struct timeval *, const struct timeval *);