Fix signedness error in 9f225e992 for gcc

Fix signedness error in 9f225e992 for gcc

The first argument of vshrq_n_s8 needs to be a signed vector type,
but it was passed unsigned. Clang is more lax with conversion, but
gcc needs a cast.

Fix by me, tested by Masahiko Sawada

Per buildfarm members splitfin, batta, widowbird, snakefly, parula,
massasauga

Discussion: https://postgr.es/m/20240306074106.mg6w4koohdlworbs%40alap3.anarazel.de