Added VEX git mirror as submodule

- libVEX is kept in a separate svn repository and integrated to
  Valgrind repository via svn:external mechanism
- vex svn address: svn://valgrind.org/vex
- Adding it as a git submodule will make it pretty simpler
  to maintain, considering future releases of valgrind/vex/freya

Signed-off-by: Nick Diego Yamane <nick.yamane@openbossa.org>