credui: Constify a variable

[Andrew Talbot]

Changelog:
    credui: Constify a variable.

diff --git a/dlls/credui/credui_main.c b/dlls/credui/credui_main.c
index 7e3c8a0..a6bbde6 100644
--- a/dlls/credui/credui_main.c
+++ b/dlls/credui/credui_main.c
@@ -138,7 +138,7 @@ struct cred_dialog_params
     BOOL fBalloonTipActive;
 };
 
-static void CredDialogFillUsernameCombo(HWND hwndUsername, struct cred_dialog_params *params)
+static void CredDialogFillUsernameCombo(HWND hwndUsername, const struct cred_dialog_params *params)
 {
     DWORD count;
     DWORD i;