extract-headers.sh: Detect platform version from version_defaults.mk

Now extract-headers.sh only needs the source and destination paths,
the version can be determined automatically from AOSP/CM sources.

Signed-off-by: Thomas Perl <thomas.perl@jolla.com>
Merge pull request #154 from thp/autodetect-header-version

extract-headers.sh: Detect platform version from version_defaults.mk