Fix build with libc++

libc++ does not include <functional> and <stdlib.h> transitively.
--- a/src/roctracer/memory_pool.h
+++ b/src/roctracer/memory_pool.h
@@ -28,6 +28,7 @@
 #include <cstdlib>
 #include <cstddef>
 #include <cstring>
+#include <functional>
 #include <future>
 #include <mutex>
 #include <type_traits>
--- a/src/roctracer/roctracer.cpp
+++ b/src/roctracer/roctracer.cpp
@@ -32,6 +32,7 @@
 #include <unistd.h>
 
 #include <atomic>
+#include <functional>
 #include <mutex>
 #include <stack>
 #include <type_traits>
--- a/test/hsa/copy.cpp
+++ b/test/hsa/copy.cpp
@@ -24,6 +24,7 @@
 
 #include <fcntl.h>
 #include <assert.h>
+#include <stdlib.h>
 #include <stdio.h>
 #include <string.h>
 #include <unistd.h>