Enable strict explicit mode (#1877)

diff --git a/kotlinx-coroutines-core/common/src/channels/Produce.kt b/kotlinx-coroutines-core/common/src/channels/Produce.kt
index 24fd399..1b1581a 100644
--- a/kotlinx-coroutines-core/common/src/channels/Produce.kt
+++ b/kotlinx-coroutines-core/common/src/channels/Produce.kt
@@ -22,7 +22,7 @@
      * All the [SendChannel] functions on this interface delegate to
      * the channel instance returned by this property.
      */
-    val channel: SendChannel<E>
+    public val channel: SendChannel<E>
 }
 
 /**