make xvideo-tests depends on virtual/libx11, not libx11 !

because libx11 breaks the build by removing Xlib.h from stagging.
whereas virtual/libx11 resolves to dietxlib that is okay.


git-svn-id: https://svn.o-hand.com/repos/poky/trunk@1991 311d38ba-8fff-0310-9ca6-ca027cbcb966