print/pdfmixtool: Fix build after ImageMagick7 7.1.1 update.
iAfter the update to ImageMagick7 7.1.1, build would fail and complain about
MAGICKCORE_HDRI_ENABLE not being defined. While pkgconf clearly propagates these
additional CFLAGS, the CMakeLists.txt of pdfmixtool does not use them.
PR: 276362
Reported by: dev@submerge.ch (maintainer)