Fix google-explicit-constructor warnings.

Bug: 28341362
Change-Id: I4504e98a8db31e0edcbe63c23f9af43eb13e9d86
diff --git a/adb/fdevent.cpp b/adb/fdevent.cpp
index 902548e..04cd865 100644
--- a/adb/fdevent.cpp
+++ b/adb/fdevent.cpp
@@ -55,7 +55,7 @@
   fdevent* fde;
   adb_pollfd pollfd;
 
-  PollNode(fdevent* fde) : fde(fde) {
+  explicit PollNode(fdevent* fde) : fde(fde) {
       memset(&pollfd, 0, sizeof(pollfd));
       pollfd.fd = fde->fd;