Futex: turn "int" into "int32_t" for portability

Even though int is 32-bit on all architectures supported by liburcu so
far, make it future-proof by uint a int32_t, which enforces the same
type width used by the system call in the kernel.

Using int32_t and not uint32_t to make comparison with 0 more
straightforward.

Reported-by: Darren Hart <dvhart@linux.intel.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>