Fix mismatch in function signature in json.cpp

Author: xzyfer

src/json.cpp

@@ -357,8 +357,8 @@ static void to_surrogate_pair(uint32_t unicode, uint16_t *uc, uint16_t *lc)
   *lc = (n & 0x3FF) | 0xDC00;
 }
 
-bool is_space        (const char *c);
-bool is_digit        (const char *c);
+static bool is_space        (const char& c);
+static bool is_digit        (const char& c);
 static bool parse_value     (const char **sp, JsonNode        **out);
 static bool parse_string    (const char **sp, char            **out);
 static bool parse_number    (const char **sp, double           *out);