Close memory leak in lexer.

Simply call talloc_strdup rather than strdup, (using the talloc_parent
of our 'state' object, (known here as yyextra).

This fix now makes glsl-orangebook-ch06-bump.frag 99.97% leak free:

	total heap usage: 55,623 allocs, 55,609 frees

Only 14 missing frees now.

glsl_lexer.lpp

@@ -312,7 +312,9 @@ highp		return HIGHP;
 precision	return PRECISION;
 
 [_a-zA-Z][_a-zA-Z0-9]*	{
-			    yylval->identifier = strdup(yytext);
+			    struct _mesa_glsl_parse_state *state = yyextra;
+			    void *ctx = talloc_parent(state);	
+			    yylval->identifier = talloc_strdup(ctx, yytext);
 			    return IDENTIFIER;
 			}