[ios] Fix compilation of "chrome" for catalyst environment

The constant is only used in a block that is not compiled when
building for catalyst, so move the definition behind an #if
check too.

Bug: none
Change-Id: Icdec2698d462d6426ce321479578e76736dcde44
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3474691
Auto-Submit: Sylvain Defresne <sdefresne@chromium.org>
Reviewed-by: Robbie Gibson <rkgibson@google.com>
Commit-Queue: Robbie Gibson <rkgibson@google.com>
Cr-Commit-Position: refs/heads/main@{#973003}

ios/chrome/browser/ui/passwords/password_suggestion_coordinator.mm

@@ -26,7 +26,9 @@
 #endif
 
 namespace {
+#if !TARGET_OS_MACCATALYST
 constexpr CGFloat preferredCornerRadius = 20;
+#endif  // TARGET_OS_MACCATALYST
 }  // namespace
 
 @interface PasswordSuggestionCoordinator () <