Fix missing declarations of altivec variant of bilinear filter functions

[belegdol]

This "fixes" #14473 in that it mame compiles. Unfortunately, the validation fails with 7967 errors like:

   Error testing rgbaint_t::bilinear_filter_rgbaint get_a32() = 5799936 (expected 78)
   Error testing rgbaint_t::bilinear_filter_rgbaint get_r32() = 5963776 (expected 236)
   Error testing rgbaint_t::bilinear_filter_rgbaint get_g32() = 6127616 (expected 109)
   Error testing rgbaint_t::bilinear_filter_rgbaint get_b32() = 3801088 (expected 40)
   Error testing rgbaint_t::bilinear_filter get_a32() = 231 (expected 169)
   Error testing rgbaint_t::bilinear_filter get_r32() = 227 (expected 174)
   Error testing rgbaint_t::bilinear_filter get_g32() = 1 (expected 96)
   Error testing rgbaint_t::bilinear_filter get_b32() = 144 (expected 136)
   Error testing rgbaint_t::bilinear_filter_rgbaint get_a32() = 7569408 (expected 169)
   Error testing rgbaint_t::bilinear_filter_rgbaint get_r32() = 7438336 (expected 174)
   Error testing rgbaint_t::bilinear_filter_rgbaint get_g32() = 32768 (expected 96)
   Error testing rgbaint_t::bilinear_filter_rgbaint get_b32() = 4718592 (expected 136)
   Error testing rgbaint_t::bilinear_filter get_a32() = 85 (expected 198)
   Error testing rgbaint_t::bilinear_filter get_r32() = 115 (expected 52)
   Error testing rgbaint_t::bilinear_filter get_g32() = 7 (expected 32)
   Error testing rgbaint_t::bilinear_filter get_b32() = 114 (expected 49)
   Error testing rgbaint_t::bilinear_filter_rgbaint get_a32() = 2785280 (expected 198)
   Error testing rgbaint_t::bilinear_filter_rgbaint get_r32() = 3768320 (expected 52)
   Error testing rgbaint_t::bilinear_filter_rgbaint get_g32() = 229376 (expected 32)
   Error testing rgbaint_t::bilinear_filter_rgbaint get_b32() = 3735552 (expected 49)
   Error testing rgbaint_t::bilinear_filter get_a32() = 45 (expected 91)
   Error testing rgbaint_t::bilinear_filter get_r32() = 186 (expected 177)
   Error testing rgbaint_t::bilinear_filter get_g32() = 98 (expected 139)
   Error testing rgbaint_t::bilinear_filter get_b32() = 8 (expected 64)
   Error testing rgbaint_t::bilinear_filter_rgbaint get_a32() = 1474560 (expected 91)
   Error testing rgbaint_t::bilinear_filter_rgbaint get_r32() = 6094848 (expected 177)
   Error testing rgbaint_t::bilinear_filter_rgbaint get_g32() = 3211264 (expected 139)
   Error testing rgbaint_t::bilinear_filter_rgbaint get_b32() = 262144 (expected 64)

I do not know whether this is an issue with the fix or with changes from 6d4ccfd.