Mark skcms.h as exporting src/skcms_public.h for include-cleaner

skcms.h is the public umbrella header, but it only forwards to
src/skcms_public.h where the API is actually declared. Tools that enforce
include-what-you-use (clang-tidy misc-include-cleaner, clangd) therefore
report every skcms_* use in client code as not directly provided by any
included header, even though skcms.h is the supported include.

Wrap the forwarding include in IWYU begin_exports/end_exports pragmas so
skcms.h is treated as the provider of the public API. The block form is
used rather than a trailing pragma because downstream import tooling
normalizes the NO_G3_REWRITE line and drops any other trailing comment.
No functional change.

Change-Id: I4b374e60aa94421863fcbf1adbb8dd98409d91c4
Reviewed-on: https://skia-review.googlesource.com/c/skcms/+/1397756
Commit-Queue: Sam Hasinoff <hasinoff@google.com>
Reviewed-by: Julia Lavrova <jlavrova@google.com>
diff --git a/skcms.h b/skcms.h
index 7a9d4c1..70bf2d1 100644
--- a/skcms.h
+++ b/skcms.h
@@ -7,4 +7,6 @@
 
 #pragma once
 
+// IWYU pragma: begin_exports
 #include "src/skcms_public.h"  // NO_G3_REWRITE
+// IWYU pragma: end_exports