build: sort sources alphabetically

The header files in the sources list are sorted alphabetically but not
the c++ source code files. This commit sorts the c++ source code file
names.

PR-URL: https://github.com/nodejs/node/pull/10892


Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Johan Bergström <bugs@bergstroem.nu>
Reviewed-By: James M Snell <jasnell@gmail.com>