imgproc: rename sumpixels.avx512_skx.{cpp,hpp}

diff --git a/modules/imgproc/src/sumpixels.avx512_skx.cpp b/modules/imgproc/src/sumpixels.avx512_skx.hpp
similarity index 100%
rename from modules/imgproc/src/sumpixels.avx512_skx.cpp
rename to modules/imgproc/src/sumpixels.avx512_skx.hpp