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/json_utils.cpp b/extensions/openpower-pels/json_utils.cpp
index f95f406..b2442ce 100644
--- a/extensions/openpower-pels/json_utils.cpp
+++ b/extensions/openpower-pels/json_utils.cpp
@@ -23,8 +23,8 @@
 
 #include <cstring>
 #include <filesystem>
+#include <fstream>
 #include <optional>
-#include <sstream>
 #include <string>
 
 namespace openpower