xgl-generate.py: add IcdGetProcAddrSubcommand
xgl-generate.py: add IcdGetProcAddrSubcommand

It can generate xglGetProcAddr() for ICDs.

v2: remove an extraneous semicolon

KhronosGroup/Vulkan-Tools,KhronosGroup/Vulkan-Tools,KhronosGroup/Vulkan-Tools,KhronosGroup/Vulkan-Tools