protoapi: fix trivial deadlock

Change-Id: I7779711d9759aa31f479847b8ffd30f1d8cb3f1d
Reviewed-on: https://go-review.googlesource.com/c/152019
Reviewed-by: Damien Neil <dneil@google.com>

protoapi/api.go

@@ -238,5 +238,5 @@ func (m extensionMap) Lock() {
 	globalLock.Lock()
 }
 func (m extensionMap) Unlock() {
-	globalLock.Lock()
+	globalLock.Unlock()
 }