Test only for KHR header

Raspberry Pi toolchain from yocto uses older khr headers that do not include
defines for KHRONOS_FALSE and KHRONOS_TRUE, but can still be used to build
qtwebengine correctly.

Change-Id: Ia7a6420ba142141ebb16d880f989eb56f892f1a3
Reviewed-by: Joerg Bornemann <joerg.bornemann@theqtcompany.com>