cmake: skip checks for host paths on cross-compilation (5897)

cmake/OpenCVFindLibsVideo.cmake

@@ -214,7 +214,7 @@ if(WITH_FFMPEG)
     CHECK_INCLUDE_FILE(libavformat/avformat.h HAVE_GENTOO_FFMPEG)
     CHECK_INCLUDE_FILE(ffmpeg/avformat.h HAVE_FFMPEG_FFMPEG)
     if(NOT HAVE_GENTOO_FFMPEG AND NOT HAVE_FFMPEG_FFMPEG)
-      if(EXISTS /usr/include/ffmpeg/libavformat/avformat.h OR HAVE_FFMPEG_SWSCALE)
+      if((NOT CMAKE_CROSSCOMPILING AND EXISTS /usr/include/ffmpeg/libavformat/avformat.h) OR HAVE_FFMPEG_SWSCALE)
         set(HAVE_GENTOO_FFMPEG TRUE)
       endif()
     endif()