src/util/futex.h: fix build on 32-bit architectures using 64-bit time_t

src/util/futex.h: fix build on 32-bit architectures using 64-bit time_t

Fix the following build failure on 32-bit architectures using 64-bit
time_t (e.g. riscv32):

../src/util/futex.h: In function 'sys_futex':
../src/util/futex.h:39:19: error: 'SYS_futex' undeclared (first use in this function); did you mean 'sys_futex'?
   39 |    return syscall(SYS_futex, addr1, op, val1, timeout, addr2, val3);
      |                   ^~~~~~~~~
      |                   sys_futex

Fixes:
 - http://autobuild.buildroot.org/results/692700a5f967760a0b8cd358b1712f1d5a7b681e

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/12496>