package/qt5/qt5base: fix build on riscv32

Fix the following build failure on riscv32:

In file included from thread/qmutex_linux.cpp:45,
                 from thread/qmutex.cpp:804:
thread/qfutex_p.h: In function 'int QtLinuxFutex::_q_futex(int*, int, int, quintptr, int*, int)':
thread/qfutex_p.h:116:30: error: '__NR_futex' was not declared in this scope; did you mean '_q_futex'?
  116 |         int result = syscall(__NR_futex, addr, op | FUTEX_PRIVATE_FLAG, val, val2, addr2, val3);
      |                              ^~~~~~~~~~
      |                              _q_futex

Fixes:
 - http://autobuild.buildroot.org/results/ffedfc000029072d5d724e98ab4551fe973658ce



Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>