vkjson: Fix include paths and clang errors

vkjson: Fix include paths and clang errors

Fixes issues discovered when trying to build vkjson as part of the
Android platform build system and a Clang 3.8-based compiler.

(a) Vulkan includes should be "<vulkan/...>".

(b) Clang issues a "missing field initializer" warning for "= {0}"
    in C++. This is arguably incorrect, but "= {}" is defined to
    recursively zero-initialize primitive types, so just use that.

(c) Clang correctly issues an error when a template parameter default
    value is redefined.