Update README for GL/glx.h dependency on Linux
Update README for GL/glx.h dependency on Linux

(cherry picked from commit 4260ae4481ab13888606eb8e1ebdeb3391691a57)

Change-Id: I77edfe92d9687d92748004be948e456ed8c8d613

KhronosGroup/VK-GL-CTS,KhronosGroup/VK-GL-CTS,KhronosGroup/Vulkan-CTS,KhronosGroup/VK-GL-CTS,KhronosGroup/VK-GL-CTS,KhronosGroup/Vulkan-CTS,KhronosGroup/VK-GL-CTS,KhronosGroup/Vulkan-CTS,KhronosGroup/Vulkan-CTS,KhronosGroup/VK-GL-CTS,KhronosGroup/Vulkan-CTS,KhronosGroup/Vulkan-CTS