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>