gdb/solib: move solist.h content to solib.h

I don't think that the file solist.h is useful.  It would make sense to
have `struct solib` in solib.h.  And then, all that would remain is
`struct solib_ops` and some solib-related function declarations.  So,
move it all to solib.h.

Change-Id: I20ecf19787c378066f2c7a6a8a737c1db7c55d9a
Reviewed-By: Guinevere Larsen <guinevere@redhat.com>