Convert over to uint32_t for AttemptsLeft

I missed the fact that the type for the reboot attempts property
changed when code was refactored to the new
xyz.openbmc_project.Control.Boot.RebootAttempts object

Resolves openbmc/openbmc#2178

Change-Id: I297d427b6fa5b9dc0077aab437548d53603c835f
Signed-off-by: Andrew Geissler <andrewg@us.ibm.com>
diff --git a/ext_interface.hpp b/ext_interface.hpp
index deb21d8..de34fef 100644
--- a/ext_interface.hpp
+++ b/ext_interface.hpp
@@ -8,4 +8,4 @@
  *
  * @return Number of boot attempts left
  **/
-int getBootCount();
+uint32_t getBootCount();