symtab reader: use C++11 `using` syntax instead of typedefs

That is to increase readability and to incrementally modernize the code base.

	* abg-symtab-reader.h: replace typedefs by corresponding using
	  declarations.

Change-Id: Iafb1e83e0ca8b6d27ade94b3cd7ebb8c2540393a
Signed-off-by: Matthias Maennich <maennich@google.com>