Add header that declares atomic primitives

It fixes a compilation issue:

/fdevent/fdevent_poll.cpp -o core/adb/fdevent/fdevent_poll.cpp.o
In file included from core/adb/fdevent/fdevent_poll.cpp:20:
In file included from core/adb/fdevent/fdevent_poll.h:29:
core/adb/fdevent/fdevent.h:106:10: error: no template named 'atomic' in namespace 'std'
    std::atomic<bool> terminate_loop_ = false;
    ~~~~~^

Change-Id: I5305a69a50e2638f514c5a7d47a9612b8ee38c69

adb/fdevent/fdevent.h

@@ -20,6 +20,7 @@
 #include <stddef.h>
 #include <stdint.h>
 
+#include <atomic>
 #include <chrono>
 #include <deque>
 #include <functional>