gdb: remove unused imports in solib*.[ch]

I'm starting to work on these files, I thought it would be a good time
to remove unused imports.  These were identified by
include-what-you-use.  Tested by rebuilding.

Change-Id: I3eaf3fa0ea3506c7ecfbc8ecff5031433b1dadb8
Reviewed-By: John Baldwin <jhb@FreeBSD.org>
8 files changed