Revert "Revert "Fix tests/plugin/windc.c""

windc doesn't link against libkdc, so it doesn't need to check its instance;
however, we will continue to export kdc_get_instance() from libkdc in case
other windc plugins do link against it.

tests/plugin/windc.c

@@ -98,9 +98,7 @@ windc_plugin_load(krb5_context context,
 static uintptr_t
 windc_get_instance(const char *libname)
 {
-    if (strcmp(libname, "kdc") == 0)
-	return kdc_get_instance(libname);
-    else if (strcmp(libname, "hdb") == 0)
+    if (strcmp(libname, "hdb") == 0)
 	return hdb_get_instance(libname);
     else if (strcmp(libname, "krb5") == 0)
 	return krb5_get_instance(libname);