Add #pragma once to all header files

[inequation]

I'm trying to use detex as an inline-included library (i.e. by #including source .c files into a translation unit of my own) to circumvent library building issues. I've encountered a problem where bits.h would be included from several files, causing an error of redefinition of the struct and the functions in it.

This is a simple and non-invasive change that fixes it. #pragma once is functionally equivalent to header guards and has wide support from compilers all across the board. It does not impact any other aspect of the project.