package/ffmpeg: altivec needs VSX with little endian

Fix the following build failure:

In file included from libavcodec/ppc/audiodsp.c:31:
libavcodec/ppc/audiodsp.c: In function 'scalarproduct_int16_altivec':
./libavutil/ppc/util_altivec.h:123:5: error: implicit declaration of function 'vec_vsx_ld'; did you mean 'vec_vslh'? [-Werror=implicit-function-declaration]
  123 |     vec_vsx_ld(offset, b)
      |     ^~~~~~~~~~

Fixes:
 - http://autobuild.buildroot.org/results/b772d285f978ff9bc3b07872d009633c943f20b1

VSX is indeed an extension to AltiVec, so havinf VSX implies having
AltiVec [0], so we can condition he altivec support on LE ,on VSX being
available.

To be noted, however, is that ffmpeg has a configre switch dedicated to
VSX: --enable-vsx. We do not use it add support for that here, as we are
just fixing the AltiVec support. Adding VSX configure flag is left as an
excercise for a future feature addition.

[0] https://en.wikipedia.org/wiki/AltiVec#VSX_(Vector_Scalar_Extension)



Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
[yann.morin.1998@free.fr:
  - add comment in .mk
  - exend commit log to explain VSX implies AltiVec
]
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>