Add yylex prototype

[ryandesign]

Fixes:

conf.tab.c:1429:16: error: implicit declaration of function 'yylex' is invalid in C99 [-Werror,-Wimplicit-function-declaration]
      yychar = YYLEX;
               ^
conf.tab.c:745:16: note: expanded from macro 'YYLEX'
# define YYLEX yylex (&yylval, &yylloc)
               ^

I don't know bison and I do not know if this is the best fix for this issue. On the bison mailing list Paul Eggert said that the prototype should be

in an include file that both conf.lex and conf.y include so that they both agree with the declaration

and I have not attempted to do that here so you may want to do that instead.

I have also read about a define called YY_DECL; I don't know if that would be relevant in this circumstance.

I figure since you know bison well enough to have written a program using it, you will know how best to resolve this issue.