Remove pragma once from amalgamated source file

[richardhozak]

This prevents GCC warning #pragma once in main file which cannot be disabled (https://gcc.gnu.org/bugzilla/show_bug.cgi?id=89808).

Note that you can disable this when using clang with -Wno-pragma-once-outside-header.

See more: https://stackoverflow.com/questions/56563679/gcc-precompiled-header-pragma-once-in-main-file

[pplux]

ah! indeed, thanks I will have a look at it, sorry I did not see the issue being there :)