Fixes compilation when bimg is compiled as amalgamated

[pplux]

The headers is included twice and redefines strucs and macros and fails to compile... added a tiny pragma once to avoid this.

[bkaradzic]

Fixed it here: https://github.com/bkaradzic/bimg/commit/aa48e8fe53731c71d04d20abf610db3e13f9399f