converters/simdutf: fix build on powerpc64

In file included from /wrkdirs/usr/ports/converters/simdutf/work/simdutf-8.2.0/src/simdutf.cpp:24:
In file included from /wrkdirs/usr/ports/converters/simdutf/work/simdutf-8.2.0/src/simdutf/ppc64.h:34:
/wrkdirs/usr/ports/converters/simdutf/work/simdutf-8.2.0/src/simdutf/ppc64/simd.h:22:37: fatal error: use of 'long long' with '__vector' requires VSX support (available on POWER7 or later) to be enabled

22 | using vec_u64_t = __vector unsigned long long;
   |                                     ^

15 warnings and 1 error generated.