hwfeatures: restore "intel-fast-vpgather" as dummy entry

* src/hwfeatures.c (hwflist) [HAVE_CPU_ARCH_X86]: Add "intel-fast-vpgather"
HW feature string without mapping to any HWF flag.

Removing "intel-fast-vpgather" caused API breakage, where
gcry_disable_hw_feature() with "intel-fast-vpgather" started returning
error.

• Signed-off-by: Jussi Kivilinna <jussi.kivilinna@iki.fi>