Add missing call to obstack_init for ada-lang.c::cache_space

Add missing call to obstack_init for ada-lang.c::cache_space

This bit was meant to be merged with the following patch:

    commit 3d9434b5ddc141178be232048e3d944955cc1f91
    Subject: [Ada] Add a symbol lookup cache

... but I forgot :-(. This causes the cache to be undefined, and
with a bit of (bad) luck:

    % gdb
    (gdb) set lang ada
    (gdb) set $xxx := 1
    [SEGV]

gdb/ChangeLog:

        * ada-lang.c (_initialize_ada_language): Initialize
        cache_space obstack.