Add *~ to .gitignore

Some tools make backups of files and traditionally name them by
appending a '~' to the original file name.  Those files can be ignored.

.gitignore

@@ -1,4 +1,5 @@
 # Ignored files
+*~
 *.suo
 *.sdf
 *.opensdf
@@ -18,4 +19,4 @@ Documentation/html
 Build
 *.aps
 *.opendb
-BsEngineConfig.h
+BsEngineConfig.h