Add build.zig

[fjebaker]

tinyxml2 is amazing and I want to use it in my Zig projects.

The best way for a Zig programmer to be able to do that is if tinyxml2 has a build.zig file in the source tree, which means downstream Zig users can use the package manager to easily integrate tinyxml2 into their projects.

Other benefits include:

• Another trivial way to build (zig build to build and zig build test to build and run the tests).
• Cross compilation for free (e.g. zig build -Dtarget=arm-linux-muslabi).

(Additional build systems are costly to maintain, and tend to bit-rot. They are being removed over time.)

I completely get that. If this is deemed irrelevant, please just close the PR. For my own projects I can depend on my fork, but I thought I would contribute back as other Zig users might also want to use tinyxml2.