Move std::experimental::nullopt_t to std

Now that we're on std::optional, use std::nullopt_t instead.

Change-Id: I81c959f3696746089b3177ed1af5f5de89e3f0f0
Signed-off-by: Ed Tanous <ed.tanous@intel.com>
diff --git a/src/utils.cpp b/src/utils.cpp
index 27c25de..8dac5ab 100644
--- a/src/utils.cpp
+++ b/src/utils.cpp
@@ -241,15 +241,14 @@
 }
 
 ServiceCache::ServiceCache(const std::string& intf, const std::string& path) :
-    intf(intf), path(path), cachedService(std::experimental::nullopt),
-    cachedBusName(std::experimental::nullopt)
+    intf(intf), path(path), cachedService(std::nullopt),
+    cachedBusName(std::nullopt)
 {
 }
 
 ServiceCache::ServiceCache(std::string&& intf, std::string&& path) :
-    intf(std::move(intf)), path(std::move(path)),
-    cachedService(std::experimental::nullopt),
-    cachedBusName(std::experimental::nullopt)
+    intf(std::move(intf)), path(std::move(path)), cachedService(std::nullopt),
+    cachedBusName(std::nullopt)
 {
 }
 
@@ -265,8 +264,8 @@
 
 void ServiceCache::invalidate()
 {
-    cachedBusName = std::experimental::nullopt;
-    cachedService = std::experimental::nullopt;
+    cachedBusName = std::nullopt;
+    cachedService = std::nullopt;
 }
 
 sdbusplus::message::message