winhttp/tests: Constify a character string.

dlls/winhttp/tests/winhttp.c

@@ -292,7 +292,7 @@ static void test_SendRequest (void)
     static const WCHAR test_file[] = {'/','p','o','s','t','t','e','s','t','.','p','h','p',0};
     static const WCHAR test_verb[] = {'P','O','S','T',0};
     static CHAR post_data[] = "mode=Test";
-    static CHAR test_post[] = "mode => Test\\0\n";
+    static const char test_post[] = "mode => Test\\0\n";
 
     header_len = -1L;
     total_len = optional_len = sizeof(post_data);