starter: Add the correct keywords header file to EXTRA_DIST

starter: Add the correct keywords header file to EXTRA_DIST

The fix for gperf in 0ae19f0ced8d added the generated header to
EXTRA_DIST but that's already added to the distribution because it is
contained in *_SOURCES, what was not added, though, was the .h.in file.

Also fixes the reference to the header file in the .c rule here and for
stroke in out-of-tree builds.

Fixes: 0ae19f0ced8d ("configure: Fix gperf length parameter determination")