Set THREADS_PREFER_PTHREAD_FLAG to YES

We have to link to -pthread instead -lpthread to get libatomic linked
on riscv64.

Signed-off-by: David Abdurachmanov <david.abdurachmanov@sifive.com>

qt5-qtwebkit.spec

@@ -161,6 +161,9 @@ cmake -DPORT=Qt \
 %ifarch s390 s390x ppc %{power64}
        -DUSE_SYSTEM_MALLOC=ON \
 %endif
+%ifarch riscv64
+       -DTHREADS_PREFER_PTHREAD_FLAG:BOOL=YES \
+%endif
 %if 0%{?docs}
        -DGENERATE_DOCUMENTATION=ON \
 %endif