Update third_party/wuffs version

Wuffs ships as a "single file C library". Previously, that single file
was a .h file. Now it is a .c file.

The contents of the file are practically the same. The different file
name extension means that the build system (GN/ninja) will treat that
file as a .c file, or "something that generates code", and not merely a
.h file, or "something #include'd by things that generate code".

This should hopefully fix mysterious linker errors when updating the
third_party/wuffs checkout results in the builder doing no work, since
no .c files changed.

Bug: skia:8235
Change-Id: I451e251d8cd23f3f0db359bbe03caa429c00fcea
Reviewed-on: https://skia-review.googlesource.com/c/180420
Reviewed-by: Leon Scroggins <scroggo@google.com>
Commit-Queue: Leon Scroggins <scroggo@google.com>
4 files changed