Move struct buildsym_compunit to buildsym.h

Move struct buildsym_compunit to buildsym.h

This moves struct buildsym_compunit to buildsym.h.  Now that the
members are private, and it no longer affects any global state in
buildsym.c, an instance can be used directly for symtab creation.

gdb/ChangeLog
2018-07-20  Tom Tromey  <tom@tromey.com>

	* buildsym.h (struct buildsym_compunit): Move from buildsym.c.
	* buildsym.c (struct buildsym_compunit): Move to buildsym.h.
	(buildsym_compunit::buildsym_compunit)
	(buildsym_compunit::~buildsym_compunit)
	(buildsym_compunit::get_macro_table): Define.