Rename QUICHE_EXPORT_PRIVATE to QUICHE_EXPORT

We do not distinguish between those in practice, and this is causing confusion, so we should stick to one, and QUICHE_EXPORT is shorter.

PiperOrigin-RevId: 482823752
diff --git a/quiche/balsa/simple_buffer.h b/quiche/balsa/simple_buffer.h
index fe81e33..96fd25f 100644
--- a/quiche/balsa/simple_buffer.h
+++ b/quiche/balsa/simple_buffer.h
@@ -18,7 +18,7 @@
 // SimpleBuffer stores data in a contiguous region.  It can grow on demand,
 // which involves moving its data.  It keeps track of a read and a write
 // position.  Reading consumes data.
-class QUICHE_EXPORT_PRIVATE SimpleBuffer {
+class QUICHE_EXPORT SimpleBuffer {
  public:
   SimpleBuffer() = default;
   // Create SimpleBuffer with at least `size` reserved capacity.