Added GPL headers to some of the benchmarks for the WCET set.

    * src/cnt/cnt.c: Added GPL header
    * src/compress/compress.c: Added GPL header
    * src/duff/duff.c: Added GPL header
    * src/edn/edn.c: Added GPL header
    * src/matmult/matmult.c: Added GPL header
Merge pull request #24 from jpallister/gpl_headers

Added GPL headers to some of the benchmarks for the WCET set.