mod_dav: Follow up to r1897156: Fix warning.

mod_dav: Follow up to r1897156: Fix warning.

In file included from mod_dav.c:51:
mod_dav.c: In function ‘uripath_is_canonical’:
mod_dav.c:774:38: error: passing argument 1 of ‘ap_strchr’ discards ‘const’ qualifier from pointer target type [-Werror=discarded-qualifiers]
  774 |             dot_pos = strchr(dot_pos + 1, '.')) {
      |                              ~~~~~~~~^~~
/home/travis/build/apache/httpd/include/httpd.h:2469:34: note: in definition of macro ‘strchr’
 2469 | # define strchr(s, c)  ap_strchr(s,c)
      |                                  ^
/home/travis/build/apache/httpd/include/httpd.h:2457:36: note: expected ‘char *’ but argument is of type ‘const char *’
 2457 | AP_DECLARE(char *) ap_strchr(char *s, int c);
      |                              ~~~~~~^

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1897183 13f79535-47bb-0310-9956-ffa450edef68