Add static_asserts for the sizes of space-critical structs

Add static_asserts for the sizes of space-critical structs

Specifically the three structs mentioned in symtab.h:
- general_symbol_info
- symbol
- partial_symbol

This ensures that those structs won't accidentally get bigger.

gdb/ChangeLog:

2019-11-04  Christian Biesinger  <cbiesinger@google.com>

	* psympriv.h: Add static_asserts for sizeof (general_symbol_info)
	and sizeof (symbol).
	* symtab.h: Add a static_assert for sizeof (partial_symbol).

Change-Id: Idd68320aa3e79ee7cc749019724636a58ce4b9c6