Why is there this warning?

[Balaco Baco]

Take the file [file.y], compile it to generate tab.[ch] files and the 
compile it (without linking) with gcc. It gives a warning in this step. 
Can fix this warning?

Commands used and their output:

====
$ bison -vd file.y
$ gcc -Wall -c file.tab.c
file.tab.c:1349: warning: implicit declaration of function ‘yylex’
# A command that works, but it is out of scope for this message
$ gcc -Wall file.tab.o [other object files] -lfl
$
====

Is this warning something expected? I was thinking about adding yylex() 
prototype to file.y, but I do not know what it is, and I could not find 
it in documentation.

file.y
Description: Text document