multimedia/mythtv: fix build on powerpc64

Error:
libswscale/ppc/yuv2rgb_altivec.c:452:1: error: call to 'vec_xl' is ambiguous
DEFCSP420_CVT(yuv2_rgb24, out_rgb24)
^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
libswscale/ppc/yuv2rgb_altivec.c:353:38: note: expanded from macro 'DEFCSP420_CVT'

u = (vector signed char) vec_xl(0, ui);                           \
                         ^~~~~~

/usr/lib/clang/13.0.0/include/altivec.h:17562:1: note: candidate function
vec_xl(ptrdiff_t offset, const unsigned char *ptr) {
^

Additionaly, a merge of https://github.com/MythTV/mythtv/commit/8960641b32f751b8198ace9bbfc3378dac20c061 is necessary.