Fix http_parser fall-through warnings (#13)

The http_parser.c library uses legacy /* fall through */ comments to
denote intentional fall-throughs in switch statements.

However, Linux kernels build external modules with stricter checks.
Since v5.14, the build system enforces -Wimplicit-fallthrough=5, which
deprecates comment markers and requires an explicit macro (commit
b7eb335e26a9, "Makefile: Enable -Wimplicit-fallthrough for Clang").
This change causes numerous build warnings when compiling http_parser.c.

To resolve this without modifying the upstream source file, this commit
adds a sed rule to the Makefile. This rule automatically converts the
legacy comments to the kernel-preferred fallthrough; pseudo-keyword at
build time, ensuring a clean build against modern kernels.

Link: Documentation/process/deprecated.rst ("fallthrough")

Author: jserv

Makefile

@@ -38,6 +38,7 @@ http_parser.c:
 	@sed -i 's/#include <ctype.h>/#include "compat\/ctype.h"/' $@
 	@sed -i 's/#include <string.h>/#include "compat\/string.h"/' $@
 	@sed -i 's/#include <limits.h>/#include "compat\/limits.h"/' $@
+	@sed -i -E 's@/\*[[:space:]]*fall[[:space:]-]*through[[:space:]]*\*/@fallthrough;@Ig' $@
 	@echo "File $@ was patched."
 	wget -q https://raw.githubusercontent.com/nodejs/http-parser/main/http_parser.h
 	@sed -i 's/#include <stddef.h>/#include "compat\/stddef.h"/' http_parser.h