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/key.h

@@ -309,4 +309,4 @@ struct CExtKey {
 /** Check that required EC support is available at runtime */
 bool ECC_InitSanityCheck(void);
 
-#endif
+#endif // BITCOIN_KEY_H