gdb/solib: fix formatting of "info linker-namespaces" error message

Add spaces after the first period and add a period at the end, resulting
in:

    (gdb) info linker-namespaces
    ❌️ Current inferior does not support linker namespaces.  Use "info sharedlibrary" instead.

Change-Id: Ib3f1647cedcdb68852a3c63df26ea3e6f791b1b1
Approved-By: Pedro Alves <pedro@palves.net>