package/libvpx: Fix compilation when __uClinux__ is defined

<lddk.h> is a platform-specific header, which is not provided by any C
library today.

Fixes:

  http://autobuild.buildroot.net/results/ae7ba1d5979ebe39aace628fa8ff2c8f08a70a98/



Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>