[v3] add missing dg-require to tests

[Eric Botcazou]

> Tested x86_64-linux and committed to trunk.
>
>         * testsuite/30_threads/async/49668.cc: Add missing dg-require.
>         * testsuite/30_threads/packaged_task/49668.cc: Likewise.

We have a related failure on SPARC/Solaris for some time:

FAIL: 30_threads/call_once/49668.cc (test for excess errors)
WARNING: 30_threads/call_once/49668.cc compilation failed to produce executable

[...]
/nile.build/botcazou/gcc-head/sparc-sun-solaris2.8/sparc-sun-solaris2.8/libstdc\
++-v3/include/tuple:105:12: error: use of deleted 
function 'moveable::moveable(const moveable&)'
/nile.build/botcazou/gcc-head/src/libstdc++-v3/testsuite/30_threads/call_once/49668.cc

It can be reproduced with a cross-compiler targetting sparc-sun-solaris2.9 and 
the .ii file.  I can open a PR if you want.

-- 
Eric Botcazou