Fix google-explicit-constructor warnings.

Bug: 28341362
Change-Id: I84effbdfa1b9b39328a909b7f70fe17e7ee316c8
diff --git a/libc/bionic/ifaddrs.cpp b/libc/bionic/ifaddrs.cpp
index 408949c..f5b080c 100644
--- a/libc/bionic/ifaddrs.cpp
+++ b/libc/bionic/ifaddrs.cpp
@@ -59,7 +59,7 @@
   sockaddr_storage ifa_ifu;
   char name[IFNAMSIZ + 1];
 
-  ifaddrs_storage(ifaddrs** list) {
+  explicit ifaddrs_storage(ifaddrs** list) {
     memset(this, 0, sizeof(*this));
 
     // push_front onto `list`.