windows: fix path handling in the schema mount

On Windows, libyang uses semicolon as path separator.

Fixes: 0fca45cf schema mount UPDATE use all the searchdirs
See-also: a3ea193c Use semicolon as a directory delimiter on Windows
diff --git a/src/plugins_exts/schema_mount.c b/src/plugins_exts/schema_mount.c
index b823a8c..1cfea52 100644
--- a/src/plugins_exts/schema_mount.c
+++ b/src/plugins_exts/schema_mount.c
@@ -297,7 +297,7 @@
     if (searchdirs) {
         /* append them all into a single string */
         for (i = 0; searchdirs[i]; ++i) {
-            if ((rc = ly_strcat(&sdirs, "%s:", searchdirs[i]))) {
+            if ((rc = ly_strcat(&sdirs, "%s" PATH_SEPARATOR, searchdirs[i]))) {
                 goto cleanup;
             }
         }