conf/yaml-loader: fix compile warning

conf-yaml-loader.c:87:8: error: assigning to 'char *' from 'const char *' discards qualifiers [-Werror,-Wincompatible-pointer-types-discards-qualifiers]
   87 |     ep = strrchr(filename, '\\');
      |        ^ ~~~~~~~~~~~~~~~~~~~~~~~
conf-yaml-loader.c:89:12: error: assigning to 'char *' from 'const char *' discards qualifiers [-Werror,-Wincompatible-pointer-types-discards-qualifiers]
   89 |         ep = strrchr(filename, '/');
      |            ^ ~~~~~~~~~~~~~~~~~~~~~~
2 errors generated.

src/conf-yaml-loader.c

@@ -83,7 +83,7 @@ Mangle(char *string)
 static void
 ConfYamlSetConfDirname(const char *filename)
 {
-    char *ep;
+    const char *ep;
 
     ep = strrchr(filename, '\\');
     if (ep == NULL)