cmake: fix version search

Currently the cmake file uses both WAFFLE_INCLUDE_DIR and
WAFFLE_INCLUDE_DIRS, but only the DIRS version is actually defined.
cmake doesn't complain about the undefined variable, instead it treats
it as a blank string. This leads to the version never being checked.

This patch replaces DIR with DIRS, which corrects the problem.

Fixes: https://github.com/waffle-gl/waffle/issues/27
Signed-off-by: Dylan Baker <dylanx.c.baker@intel.com>

I agree with emil that the right solution in the long term is to replace this file with package config and version files is a better idea than providing a find file, but this is here and very easy to get fixed and working.

This should be suitable for maint branches as well.