Add missing includes.

Includes are transitively imported by <string> in String8.h + String16.h
but that include is being removed.

Test: pass
Change-Id: Ide5c011b40b4a4f031dd26ead08b5c8d5d299693
diff --git a/libutils/ProcessCallStack.cpp b/libutils/ProcessCallStack.cpp
index 1d9f30d..ff8b32a 100644
--- a/libutils/ProcessCallStack.cpp
+++ b/libutils/ProcessCallStack.cpp
@@ -20,6 +20,7 @@
 #include <utils/ProcessCallStack.h>
 
 #include <dirent.h>
+#include <memory>
 
 #include <utils/Printer.h>