meson: install KHR/khrplatform.h when needed

Fixes: f7d42ee7d3 "include: update GL & GLES headers (v2)"
Signed-off-by: Eric Engestrom <eric.engestrom@intel.com>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>

include/meson.build

@@ -43,7 +43,7 @@ if with_gles2
   )
 endif
 
-if with_gles1 or with_gles2 or with_egl
+if with_gles1 or with_gles2 or with_opengl or with_egl
   install_headers('KHR/khrplatform.h', subdir : 'KHR')
 endif