firmware_handler: use enum class for Session::State

Use an enum class for Session::State instead of an enum.

Signed-off-by: Patrick Venture <venture@google.com>
Change-Id: I52a4b42a65df5e218743b00b12615ea1d16b3c6a
diff --git a/firmware_handler.hpp b/firmware_handler.hpp
index 8d41ba6..1af3d04 100644
--- a/firmware_handler.hpp
+++ b/firmware_handler.hpp
@@ -59,7 +59,7 @@
     std::uint16_t flags;
 
     /** A sesion can be for an image (or tarball) or the hash. */
-    enum State
+    enum class State
     {
         open = 0,
         closed = 1,