tesseract-lang.inc: set ${S} to fix the QA warning

The ${S} is not set in the recipe cause the QA warning:
WARNING: tesseract-lang-eng: the directory ${WORKDIR}/${BP}
(/buildarea2/build/tmp/work/all-poky-linux/tesseract-lang-eng/3.02-r0/tesseract-lang-eng-3.02)
pointed to by the S variable doesn't exist - please set S within the recipe to point to where the source has been unpacked to

Signed-off-by: Yi Zhao <yi.zhao@windriver.com>