constexpr constructor for atfork_list_t

Bug: http://b/26026986
Change-Id: Iad95383a23f81eea776bebce641c555d26547b77
diff --git a/libc/bionic/pthread_atfork.cpp b/libc/bionic/pthread_atfork.cpp
index 093ffd2..2200a6c 100644
--- a/libc/bionic/pthread_atfork.cpp
+++ b/libc/bionic/pthread_atfork.cpp
@@ -45,7 +45,7 @@
 
 class atfork_list_t {
  public:
-  atfork_list_t() : first_(nullptr), last_(nullptr) {}
+  constexpr atfork_list_t() : first_(nullptr), last_(nullptr) {}
 
   template<typename F>
   void walk_forward(F f) {