externals: Update glslang revision
externals: Update glslang revision

Updated glslang to use commit
3fc1543794d1c368d690e60eaac6111c8c436aa7 from 10/20/2016.

Change-Id: I016273fbe936abd2dd5c19f5ee9ce3e6d09d72e6

KhronosGroup/Vulkan-LoaderAndValidationLayers,KhronosGroup/Vulkan-LoaderAndValidationLayers,KhronosGroup/Vulkan-LoaderAndValidationLayers,KhronosGroup/Vulkan-LoaderAndValidationLayers