storage: gcc8: standardize use of std::filesystem

This was already done repo-wide with 185b9f8b but this one spot appears
to have been missed, and was caught by gcc8:

| ../git/storagehandler.cpp:276:39: error: 'filesystem' is not a namespace-name
|      namespace fs = std::experimental::filesystem;
|                                        ^~~~~~~~~~

Change-Id: I79d70f0ba1268eae9fc76be690313a5128b59f17
Signed-off-by: Brad Bishop <bradleyb@fuzziesquirrel.com>
diff --git a/storagehandler.cpp b/storagehandler.cpp
index 5ac116c..ebd0a20 100644
--- a/storagehandler.cpp
+++ b/storagehandler.cpp
@@ -273,7 +273,7 @@
                           ipmi_request_t request, ipmi_response_t response,
                           ipmi_data_len_t data_len, ipmi_context_t context)
 {
-    namespace fs = std::experimental::filesystem;
+    namespace fs = std::filesystem;
     auto requestData =
         reinterpret_cast<const ipmi::sel::DeleteSELEntryRequest*>(request);