libxml2: Add bash dependency for ptests.

Before, running ptests on core-image-minimal would result in
an error due to missing /bin/bash:

   [ -d test   ] || ln -s ../libxml2-2.9.10/test   .
   make: /bin/bash: No such file or directory
   make: *** [Makefile:2105: runtests] Error 127

Changing the Makefile to use /bin/sh results in some of the
tests failing, so I have added the missing dependancy on bash.

Signed-off-by: Tony Tascioglu <tony.tascioglu@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>