I updated stale code in STB_C_LEX_ISWHITE

[jeng]

It looks like some stale code was left in the ifdef for STB_C_LEX_ISWHITE. The changes worked for me with this definition of isWhite in the file I'm using the library from:

#define STB_C_LEXER_IMPLEMENTATION
int myWhite(char *c){
    return (*c == ' ' || *c == '\t' || *c == '\f');
}
#define STB_C_LEX_ISWHITE(x) myWhite(x)
#include "stb_c_lexer.h"