scripts: extract_dts_includes: Fix path handling in Windows

When building in Windows, extract_dts_includes.py failed with an
exception on boards that have dts sensor bindings. This was caused by
path handling to search for i2c-device.yaml that worked in Linux, but
not in Windows.

Affected boards were disco_l475_iot1, frdm_k64f, frdm_kw41z, and
hexiwear_k64.

Signed-off-by: Maureen Helm <maureen.helm@nxp.com>