lg2: fix clang issue on nullptr_t

clang-tidy is more pedantic that nullptr_t is actually in std and
fails unless it is specified as such.

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: I74aa5a5af17fd5d4610fed4707564576c6ac22f1
diff --git a/lib/include/phosphor-logging/lg2/conversion.hpp b/lib/include/phosphor-logging/lg2/conversion.hpp
index 0c24c5b..3a81d8d 100644
--- a/lib/include/phosphor-logging/lg2/conversion.hpp
+++ b/lib/include/phosphor-logging/lg2/conversion.hpp
@@ -28,7 +28,7 @@
 concept string_like_type =
     (std::constructible_from<std::string_view, T> ||
      std::same_as<std::filesystem::path,
-                  std::decay_t<T>>)&&!std::same_as<nullptr_t, T>;
+                  std::decay_t<T>>)&&!std::same_as<std::nullptr_t, T>;
 
 /** Concept to determine if an item acts like a pointer.
  *
@@ -37,7 +37,7 @@
  */
 template <typename T>
 concept pointer_type = (std::is_pointer_v<T> ||
-                        std::same_as<nullptr_t, T>)&&!string_like_type<T>;
+                        std::same_as<std::nullptr_t, T>)&&!string_like_type<T>;
 
 /** Concept to determine if an item acts like an unsigned_integral.
  *