This CL moves build_module into an anonymous namespace which fixes up a
build warning about missing prototypes.
Change-Id: I8b2f427e32e7851c24492099103565ab0119f4fa
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/16503
Reviewed-by: Sarah Mashayekhi <sarahmashay@google.com>