kvm tools: Add WERROR to Makefile for disabling -Werror

This patch adds a WERROR variable to Makefile that can be used as follows:

  make WERROR=0

to disable '-Werror' while building. This is useful while doing 'git bisect',
for example.

Suggested-by: Ingo Molnar <mingo@elte.hu>
Cc: Cyrill Gorcunov <gorcunov@gmail.com>
Signed-off-by: Pekka Enberg <penberg@kernel.org>