bmc: move Verification declaration out of single header

Split the interface declaration from the implementation declaration to
make it more obvious what would change to support more implementations.

Signed-off-by: Patrick Venture <venture@google.com>
Change-Id: I6452ad85513dad1431ef946b62936c1cea89c263
diff --git a/firmware_handler.cpp b/firmware_handler.cpp
index c2e935a..5f9a089 100644
--- a/firmware_handler.cpp
+++ b/firmware_handler.cpp
@@ -18,6 +18,7 @@
 
 #include "image_handler.hpp"
 #include "util.hpp"
+#include "verify.hpp"
 
 #include <algorithm>
 #include <cstdint>