audio/sc3-plugins: fix build on powerpc*

In file included from /wrkdirs/usr/ports/audio/sc3-plugins/work/sc3-plugins-Version-3.13.0/source/VBAPUGens/VBAP.cpp:77:
In file included from /wrkdirs/usr/ports/audio/sc3-plugins/work/sc3-plugins-Version-3.13.0/external_libraries/nova-simd/simd_memory.hpp:26:
In file included from /wrkdirs/usr/ports/audio/sc3-plugins/work/sc3-plugins-Version-3.13.0/external_libraries/nova-simd/vec.hpp:30:
In file included from /wrkdirs/usr/ports/audio/sc3-plugins/work/sc3-plugins-Version-3.13.0/external_libraries/nova-simd/vec/vec_altivec.hpp:27:
/wrkdirs/usr/ports/audio/sc3-plugins/work/sc3-plugins-Version-3.13.0/external_libraries/nova-simd/vec/vec_int_altivec.hpp:38:23: error: non-constant-expression cannot be narrowed from type 'int' to 'unsigned int' in initializer list [-Wc++11-narrowing]

38 |         return (ivec){i, i, i, i};
   |                       ^