gl3w headers not installed

[themightyoarfish]

When my dependent project attempts to build against installed iridescence, I have the problem of missing gl3w headers:

include/iridescence/GL/glcorearb.h:82:10: fatal error: 'KHR/khrplatform.h' file not found
   82 | #include <KHR/khrplatform.h>
      |          ^~~~~~~~~~~~~~~~~~~

Because they are not getting installed. Adding

install(DIRECTORY thirdparty/gl3w/KHR/ DESTINATION ${INCLUDE_INSTALL_DIR}/KHR)

Fixes this particular problem.