add missing header end comments

- ensures a consistent usage in header files
- also add a blank line after the copyright header where missing
- also remove orphan new-lines at the end of some files

src/threadsafety.h

@@ -51,4 +51,5 @@
 #define SHARED_LOCKS_REQUIRED(...)
 #define NO_THREAD_SAFETY_ANALYSIS
 #endif  // __GNUC__
+
 #endif  // BITCOIN_THREADSAFETY_H