Use the result of converting to a unix path.

The commit 18a3195a ('Keep track of INCLUDE_DIRECTORIES as a vector
of structs.', 2012-11-19) moved the handling of includes from
cmGeneratorTarget to cmTarget, but in the process introduced this
bug.
Merge topic 'fix-include-directories-unix-path' into next

b6f6654 Use the result of converting to a unix path.