Export TotalStringViewSpanSize.

Adds QUICHE_EXPORT to the declaration of TotalStringViewSpanSize to make it visible to quiche_string_tuple_test.cc when built in Chromium.

PiperOrigin-RevId: 872540745

quiche/common/vectorized_io_utils.h

diff --git a/quiche/common/vectorized_io_utils.h b/quiche/common/vectorized_io_utils.h
index 0d07290..2a35184 100644
--- a/quiche/common/vectorized_io_utils.h
+++ b/quiche/common/vectorized_io_utils.h
@@ -14,7 +14,8 @@
 namespace quiche {
 
 // Computes the total size of all strings in the provided span.
-size_t TotalStringViewSpanSize(absl::Span<const absl::string_view> span);
+size_t QUICHE_EXPORT
+TotalStringViewSpanSize(absl::Span<const absl::string_view> span);
 
 // Copies data contained in `inputs` into `output`, up until either the `output`
 // is full or the `inputs` are copied fully; returns the actual number of bytes