Mercury.gitignore: Add Erlang .beams, as this is an

additional compiler target, forgot to add this in
https://github.com/github/gitignore/pull/1057

Mercury.gitignore

@@ -7,5 +7,6 @@ Mercury/
 *.a
 *.so
 *.dylib
+*.beams
 *.d
 *.c_date