pw_polyfill/cstddef: Add missing constexpr specifiers

The std::byte shift operators were missing the constexpr
specifiers, this adds them back in.

For reference: https://en.cppreference.com/w/cpp/types/byte

Change-Id: I3610602e575332962df3f552695352fc1f2b7af5
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/78588
Reviewed-by: Wyatt Hepler <hepler@google.com>
Pigweed-Auto-Submit: Ewout van Bekkum <ewout@google.com>
Commit-Queue: Auto-Submit <auto-submit@pigweed.google.com.iam.gserviceaccount.com>
diff --git a/pw_polyfill/standard_library_public/pw_polyfill/standard_library/cstddef.h b/pw_polyfill/standard_library_public/pw_polyfill/standard_library/cstddef.h
index cc97310..e8dfc9d 100644
--- a/pw_polyfill/standard_library_public/pw_polyfill/standard_library/cstddef.h
+++ b/pw_polyfill/standard_library_public/pw_polyfill/standard_library/cstddef.h
@@ -61,12 +61,12 @@
 constexpr byte& operator^=(byte& l, byte r) noexcept { return l = l ^ r; }
 
 template <typename I>
-inline byte& operator<<=(byte& b, I shift) noexcept {
+constexpr inline byte& operator<<=(byte& b, I shift) noexcept {
   return b = b << shift;
 }
 
 template <typename I>
-inline byte& operator>>=(byte& b, I shift) noexcept {
+constexpr inline byte& operator>>=(byte& b, I shift) noexcept {
   return b = b >> shift;
 }