Fix GetSubTree depth argument type

GetSubTree takes an integer for the depth, and not a size_t.
The upcoming mapper implementation needs it correct.

Tested:  No longer throws an exception with the fix.

Change-Id: I9bda8872bf26a67a7edf9ff818d36b13a750def0
Signed-off-by: Matt Spinler <spinler@us.ibm.com>
diff --git a/dbus.cpp b/dbus.cpp
index 70380d3..f0eb0b9 100644
--- a/dbus.cpp
+++ b/dbus.cpp
@@ -83,7 +83,7 @@
 }
 
 DbusSubtree getSubtree(sdbusplus::bus::bus& bus, const std::string& root,
-                       size_t depth, const std::string& interface)
+                       int depth, const std::string& interface)
 {
     DbusSubtree tree;