package/x11r7/xlib_libxshmfence: disable on riscv32

package/x11r7/xlib_libxshmfence: disable on riscv32

xlib_libxshmfence unconditionally uses SYS_futex which raises the
following build failure on riscv32:

xshmfence_futex.h:58:17: error: 'SYS_futex' undeclared (first use in this function); did you mean 'sys_futex'?
   58 |  return syscall(SYS_futex, addr1, op, val1, timeout, addr2, val3);
      |                 ^~~~~~~~~
      |                 sys_futex

Fixes:
 - http://autobuild.buildroot.org/results/b3523e35fde0fac04b96a6278cbc6ffdfe56f7d1

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>