forkfd: fix build with GCC 4.8 & 4.9

Prior to GCC 5.x, std::atomic_int was a typedef to __atomic_base<int>,
which meant we couldn't use it in atomic_compare_exchange that requires
pointers to std::atomic<int>. That changed in 5.x (r219790) in response
to https://gcc.gnu.org/bugzilla/show_bug.cgi?id=60940.

Easy fix, though.

Fixes: QTBUG-80896
Change-Id: I46bf1f65e8db46afbde5fffd15e1d625154f8d59
Reviewed-by: Ville Voutilainen <ville.voutilainen@qt.io>

src/3rdparty/forkfd/forkfd_c11.h

@@ -36,7 +36,7 @@
 #  define FFD_ATOMIC_ACQUIRE        std::memory_order_acquire
 #  define FFD_ATOMIC_RELEASE        std::memory_order_release
 // acq_rel & cst not necessary
-typedef std::atomic_int ffd_atomic_int;
+typedef std::atomic<int> ffd_atomic_int;
 #else
 #  include <stdatomic.h>
 #  define ffd_atomic_pointer(type)  _Atomic(type*)