PEL: Removed unused includes

The clangd LSP indicates when includes aren't used directly.

Tested:
Unit tests pass.

Change-Id: I8290fe59abd1cef4fada27aeb5adb2c45c289abb
Signed-off-by: Matt Spinler <spinler@us.ibm.com>
diff --git a/extensions/openpower-pels/callout.hpp b/extensions/openpower-pels/callout.hpp
index 7ac0727..2f50271 100644
--- a/extensions/openpower-pels/callout.hpp
+++ b/extensions/openpower-pels/callout.hpp
@@ -6,6 +6,8 @@
 #include "pel_types.hpp"
 #include "stream.hpp"
 
+#include <memory>
+
 namespace openpower
 {
 namespace pels