FindImageMagick: Find v6 include dir (#14174)

The header files of ImageMagick are now located at
<prefix>/include/ImageMagick-6 instead of <prefix>/include/ImageMagick.

Modules/FindImageMagick.cmake

@@ -81,7 +81,7 @@ function(FIND_IMAGEMAGICK_API component header)
       ${ImageMagick_INCLUDE_DIRS}
       "[HKEY_LOCAL_MACHINE\\SOFTWARE\\ImageMagick\\Current;BinPath]/include"
     PATH_SUFFIXES
-      ImageMagick
+      ImageMagick ImageMagick-6
     DOC "Path to the ImageMagick include dir."
     )
   find_library(ImageMagick_${component}_LIBRARY