gdb/command.h: Include common/scoped_restore.h

gdb/command.h: Include common/scoped_restore.h

command.h depends on scoped_restore:

  extern scoped_restore_tmpl<int> prevent_dont_repeat (void);

But doesn't include the corresponding header
("common/scoped_restore.h").  We haven't noticed a problem because
utils.h includes scoped_restore.h, and defs.h includes utils.h.

However, a patch that makes "symtab.h" include "completer.h", exposed
the issue:
 https://sourceware.org/ml/gdb-patches/2017-06/msg00023.html.

Without this fix that would break building all .o files like this:

 In file included from src/gdb/completer.h:21:0,
                  from src/gdb/symtab.h:28,
                  from src/gdb/language.h:26,
                  from src/gdb/frame.h:72,
                  from src/gdb/gdbarch.h:39,
                  from src/gdb/defs.h:636,
                  from src/gdb/top.c:20:
 src/gdb/command.h:434:8: error: ‘scoped_restore_tmpl’ does not name a type
  extern scoped_restore_tmpl<int> prevent_dont_repeat (void);
         ^
 Makefile:1911: recipe for target 'top.o' failed

because defs.h includes gdbarch.h before it includes utils.h.

gdb/ChangeLog:
2017-06-28  Pedro Alves  <palves@redhat.com>

	* command.h: Include "common/scoped_restore.h".