SL-821: Fix BugSplat attachment method override per BugSplat support.

The example code on the BugSplat documentation page
https://www.bugsplat.com/docs/platforms/os-x#configuration
omits certain essential Objective-C++ boilerplate incantations. Adding them
at least compiles successfully.

indra/newview/llappdelegate-objc.mm

@@ -194,7 +194,11 @@
 
 #if defined(LL_BUGSPLAT)
 
-@implementation BugsplatStartupManagerDelegate
+// per Geoff at BugSplat support
+@interface AppDelegate : NSObject <NSApplicationDelegate, BugsplatStartupManagerDelegate>
+@end
+
+@implementation AppDelegate
 
 - (BugsplatAttachment *)attachmentForBugsplatStartupManager:(BugsplatStartupManager *)bugsplatStartupManager {
     std::string logfile = getLogFilePathname();