src: Consistently spell whitespace without whitespace

Whitespace is spelled without a space between white and space, so
make sure to consistently spell it that way across the codebase.

Closes #6023
Reviewed-by: Daniel Stenberg <daniel@haxx.se>
Reviewed-by: Emil Engler <me@emilengler.com>

lib/http.c

@@ -1355,7 +1355,7 @@ Curl_compareheader(const char *headerline, /* line to check */
   /* pass the header */
   start = &headerline[hlen];
 
-  /* pass all white spaces */
+  /* pass all whitespace */
   while(*start && ISSPACE(*start))
     start++;