build: avoid obsolete index()

Commit 2a530a3e is not portable to mingw, which intentionally
avoids declaring the obsolete index().  See also:
https://bugzilla.redhat.com/show_bug.cgi?id=1214605

* src/util/virstring.c (virStringStripControlChars): Use strchr.
Signed-off-by: NEric Blake <eblake@redhat.com>

src/util/virstring.c

 /*
- * Copyright (C) 2012-2014 Red Hat, Inc.
+ * Copyright (C) 2012-2015 Red Hat, Inc.
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Lesser General Public
@@ -1003,7 +1003,7 @@ virStringStripControlChars(char *str)
 
     len = strlen(str);
     for (i = 0, j = 0; i < len; i++) {
-        if (index(control_chars, str[i]))
+        if (strchr(control_chars, str[i]))
             continue;
 
         str[j++] = str[i];