powerpc/crypto: Fix aes-gcm-p10 build when VSX=n

When VSX is disabled, eg. microwatt_defconfig, the build fails with:

  In function ‘enable_kernel_vsx’,
      inlined from ‘vsx_begin’ at arch/powerpc/crypto/aes-gcm-p10-glue.c:68:2,
      inlined from ‘p10_aes_gcm_crypt.constprop’ at arch/powerpc/crypto/aes-gcm-p10-glue.c:244:2:
  ...
  arch/powerpc/include/asm/switch_to.h:86:9: note: in expansion of macro ‘BUILD_BUG’
     86 |         BUILD_BUG();
        |         ^~~~~~~~~

Fix it by making the p10-aes-gcm code depend on VSX.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20230515124731.122962-1-mpe%40ellerman.id.au