nix-expr/parser: Suppress warning

We rely on `yyerror()` instead.

> The variable yynerrs contains the number of syntax errors reported so
> far.
> Normally this variable is global; but if you request a pure parser
> (see A Pure (Reentrant) Parser) then it is a local variable which only
> the actions can access.

https://www.gnu.org/software/bison/manual/html_node/Error-Reporting-Function.html

src/libexpr/parser.y

@@ -179,7 +179,12 @@ static Expr * makeCall(PosIdx pos, Expr * fn, Expr * arg) {
 
 %%
 
-start: expr { state->result = $1; };
+start: expr {
+  state->result = $1;
+
+  // This parser does not use yynerrs; suppress the warning.
+  (void) yynerrs;
+};
 
 expr: expr_function;