[Backtracing] Don't try to include <libproc.h> for iOS.

iOS doesn't have <libproc.h>.  We don't need it anyway there.

rdar://104336548

Author: al45tair

stdlib/public/SwiftShims/swift/shims/_SwiftBacktracing.h

@@ -29,7 +29,9 @@
 
 #include <CoreFoundation/CFUUID.h>
 #include <CoreFoundation/CFString.h>
+#endif
 
+#if TARGET_OS_OSX
 #include <libproc.h>
 #endif