gdb: fix build error in unittests/parallel-for-selftests.c

gdb: fix build error in unittests/parallel-for-selftests.c

We get this error when building GDB on some platforms.  I get it using
g++-10 on Ubuntu 20.04 (installed using the distro package).  It was
also reported by John Baldwin, using a clang that uses libc++.

      CXX    unittests/parallel-for-selftests.o
    cc1plus: warning: command line option '-Wmissing-prototypes' is valid for C/ObjC but not for C++
    /home/smarchi/src/binutils-gdb/gdb/unittests/parallel-for-selftests.c: In function 'void selftests::parallel_for::test(int)':
    /home/smarchi/src/binutils-gdb/gdb/unittests/parallel-for-selftests.c:53:30: error: use of deleted function 'std::atomic<int>::atomic(const std::atomic<int>&)'
       53 |   std::atomic<int> counter = 0;
          |                              ^
    In file included from /usr/include/c++/9/future:42,
                     from /home/smarchi/src/binutils-gdb/gdb/../gdbsupport/thread-pool.h:29,
                     from /home/smarchi/src/binutils-gdb/gdb/../gdbsupport/parallel-for.h:26,
                     from /home/smarchi/src/binutils-gdb/gdb/unittests/parallel-for-selftests.c:22:
    /usr/include/c++/9/atomic:755:7: note: declared here
      755 |       atomic(const atomic&) = delete;
          |       ^~~~~~
    /usr/include/c++/9/atomic:759:17: note:   after user-defined conversion: 'constexpr std::atomic<int>::atomic(std::atomic<int>::__integral_type)'
      759 |       constexpr atomic(__integral_type __i) noexcept : __base_type(__i) { }
          |                 ^~~~~~

I haven't dug to know why it does not happen everywhere, but this patch
fixes it by using the constructor to initialize the variable, rather
than the assignment operator.

Change-Id: I6b27958171bf6187f6a875657395fd10441db7e6