auxvec.h: account for AT_HWCAP2 in AT_VECTOR_SIZE_BASE

Commit 2171364d

 ("powerpc: Add HWCAP2 aux entry") introduced a new
AT_ auxv entry type AT_HWCAP2 but failed to update AT_VECTOR_SIZE_BASE
accordingly.

Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Fixes: 2171364d

 (powerpc: Add HWCAP2 aux entry)
Cc: stable@vger.kernel.org
Acked-by: Michael Neuling <michael@neuling.org>
Cc: Nishanth Aravamudan <nacc@linux.vnet.ibm.com>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>