vulkan/util: Teach gen_enum_to_str.py to parse mutliple XML files

vulkan/util: Teach gen_enum_to_str.py to parse mutliple XML files

To give the script multiple XML files, call it like so:

    gen_enum_to_str.py --xml a.xml --xml b.xml --xml c.xml ...

The script parses the XML files in the given order.

This will allow us to feed the script XML files for extensions that are
missing from the official vk.xml, such as VK_ANDROID_native_buffer.

Reviewed-by: Tapani Pälli <tapani.palli@intel.com>