Fix execution_direction's type

Fix execution_direction's type

This fixes a few build errors like these in C++ mode:

  src/gdb/reverse.c: In function ‘void exec_reverse_once(char*, char*, int)’:
  src/gdb/reverse.c:49:34: error: invalid conversion from ‘int’ to ‘exec_direction_kind’ [-fpermissive]
     enum exec_direction_kind dir = execution_direction;
				    ^
  make: *** [reverse.o] Error 1

gdb/ChangeLog:
2015-10-13  Pedro Alves  <palves@redhat.com>

	* infrun.c (restore_execution_direction): New function.
	(fetch_inferior_event): Use it instead of
	make_cleanup_restore_integer.
	(execution_direction): Change type to enum
	exec_direction_kind.
	* infrun.h (execution_direction): Likewise.