strcat.h isn't needed and it causes a conflict if StrCat is already defined.

diff --git a/base/types/expected.h b/base/types/expected.h
--- a/base/types/expected.h
+++ b/base/types/expected.h
@@ -12,7 +12,9 @@
 #include <variant>
 
 #include "base/check.h"
+#if !defined(MOZ_SANDBOX)
 #include "base/strings/strcat.h"
+#endif  // !defined(MOZ_SANDBOX)
 #include "base/strings/to_string.h"
 #include "base/types/expected_internal.h"  // IWYU pragma: export