Default initialize enum flags to 0

Default initialize enum flags to 0

... so that we don't need to do it manually, and potentially forget.
For example, this allows to do:

  my_flags flags;

  ...

  flags |= some_flag;

gdb/ChangeLog:

	* common/enum-flags.h (enum_flags::enum_flags): Initialize
	m_enum_value to 0 in default constructor.