Unreviewed, fix the Mac Catalyst build after 276581@main

Explicitly cast to `UIPasteboard`, so that the Catalyst WebCore build (that has declarations for
both `+[NSPasteboard generalPasteboard]` from AppKit as well as `+[UIPasteboard generalPasteboard]`
from UIKit) won't get confused.

* Source/WebCore/platform/ios/PlatformPasteboardIOS.mm:
(WebCore::generalUIPasteboard):
(WebCore::PlatformPasteboard::PlatformPasteboard):

Canonical link: https://commits.webkit.org/276599@main

Source/WebCore/platform/ios/PlatformPasteboardIOS.mm

@@ -59,8 +59,13 @@ @interface UIPasteboard () <AbstractPasteboard>
 
 namespace WebCore {
 
+static UIPasteboard *generalUIPasteboard()
+{
+    return static_cast<UIPasteboard *>([PAL::getUIPasteboardClass() generalPasteboard]);
+}
+
 PlatformPasteboard::PlatformPasteboard()
-    : m_pasteboard([PAL::getUIPasteboardClass() generalPasteboard])
+    : m_pasteboard(generalUIPasteboard())
 {
 }
 
@@ -70,11 +75,11 @@ @interface UIPasteboard () <AbstractPasteboard>
     if (name == Pasteboard::nameOfDragPasteboard())
         m_pasteboard = [WebItemProviderPasteboard sharedInstance];
     else
-        m_pasteboard = [PAL::getUIPasteboardClass() generalPasteboard];
+        m_pasteboard = generalUIPasteboard();
 }
 #else
 PlatformPasteboard::PlatformPasteboard(const String&)
-    : m_pasteboard([PAL::getUIPasteboardClass() generalPasteboard])
+    : m_pasteboard(generalUIPasteboard())
 {
 }
 #endif