user32/tests: Cast-qual warning fixes.

dlls/user32/tests/win.c

diff --git a/dlls/user32/tests/win.c b/dlls/user32/tests/win.c
index 3dde99a..a420e28 100644
--- a/dlls/user32/tests/win.c
+++ b/dlls/user32/tests/win.c
@@ -1056,7 +1056,7 @@
 
 /************** MDI test ****************/
 
-static const char mdi_lParam_test_message[] = "just a test string";
+static char mdi_lParam_test_message[] = "just a test string";
 
 static void test_MDI_create(HWND parent, HWND mdi_client, INT first_id)
 {