And fix a linker error (which I can't reproduce) on TLS targets, which
can use an explicit instantiation because it doesn't refer to an
unnamed closure type.
PR libstdc++/52128
* src/c++11/future.cc: Add explicit instantiation.
Tested x86_64-linux, powerpc-linux, i686-linux, x86_6
The out-of-line destructor in future.cc uses std::call_once which uses
a lambda on TLS targets. Because we compile with
-fno-implicit-templates an explicit instantiation is needed, but can't
be done because the closure type created by the lambda can't be named.
This puts the destructor inline in t
