Fix compilation with new flex (#700958).

Due to a type change [1], linuxdoc-tools won't compile with flex-2.5.36
and newer (Debian currently ships 2.5.35).

This patch works around this problem.

[1] http://flex.git.sourceforge.net/git/gitweb.cgi?p=flex/flex;a=commitdiff;h=9ba3187a537d6a58d345f2874d06087fd4050399