util/magic: fix compile warning in test

util-magic.c:443:11: error: initializing 'char *' with an expression of type 'const char *' discards
      qualifiers [-Werror,-Wincompatible-pointer-types-discards-qualifiers]
  443 |     char *str = strstr(result, "OpenDocument Text");
      |           ^     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1 error generated.

src/util-magic.c

@@ -440,7 +440,7 @@ static int MagicDetectTest07(void)
     result = MagicThreadLookup(&ctx, buffer, buffer_len);
     FAIL_IF_NULL(result);
 
-    char *str = strstr(result, "OpenDocument Text");
+    const char *str = strstr(result, "OpenDocument Text");
     FAIL_IF_NULL(str);
 
     MagicDeinitContext(ctx);