gdb: drop struct keyword when using bound_minimal_symbol

This is a simple find / replace from "struct bound_minimal_symbol" to
"bound_minimal_symbol", to make things shorter and more consisten
througout.  In some cases, move variable declarations where first used.

Change-Id: Ica4af11c4ac528aa842bfa49a7afe8fe77a66849
Reviewed-by: Keith Seitz <keiths@redhat.com>
Approved-By: Andrew Burgess <aburgess@redhat.com>