dnn: vec_neon: avoid redefinition of vcvtnq_s32_f32

clang exposes this intrinsic even in 32-bit mode, if targeting >= armv8, whereas gcc does not, see:

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95399

This fixes the android-arm build here: https://code.videolan.org/tmatth/vlc/-/pipelines/445093

requested review from @jmvalin

merged

closed