package/mesa3d: fix build on riscv32

package/mesa3d: fix build on riscv32

Fix the following build failure on 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>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>