conf: make dirname pointer const

Fixes warning:

conf-yaml-loader.c: In function ‘ConfYamlSetConfDirname’:
conf-yaml-loader.c:87:8: warning: assignment discards ‘const’ qualifier from pointer target type [-Wdiscarded-qualifiers]
   87 |     ep = strrchr(filename, '\\');
      |        ^

src/conf-yaml-loader.c

@@ -82,7 +82,7 @@ Mangle(char *string)
 static void
 ConfYamlSetConfDirname(const char *filename)
 {
-    char *ep;
+    const char *ep;
 
     ep = strrchr(filename, '\\');
     if (ep == NULL)