xkbscan: Copy, rather than assign, file name

xkbscan: Copy, rather than assign, file name

For some reason, lex decided to reduce a strcpy into an assignment,
leading to entirely justified valgrind warnings about invalid reads,
when scanFile was set to a string which may have only ever lived on the
stack of a now-exited function.

Make it a strdup() instead.

Signed-off-by: Daniel Stone <daniel@fooishbar.org>