gdb: fix completion of anonymous struct members

Completing fields inside an anonymous struct does not work.  With:

    struct commit_counters_hot {
    	union {
    		struct {
    			long owner;
    		};
    		char padding[16];
    	};
    };

I get:

    (gdb) complete print cc_hot.
    print cc_hot.padding

After this patch, I get:

    (gdb) complete print cc_hot.
    print cc_hot.owner
    print cc_hot.padding

Update break1.c to include an anonymous struct.  The tests that complete
"z_field" inside gdb.base/completion.exp would start to fail without the
fix.

Change-Id: I46b65a95ad16b0825de58dfa241777fe57acc361
Reviewed-By: Keith Seitz <keiths@redhat.com>