Revert "message: set 'get' as const"

This reverts commit 2e37e97c5e9eb4637827bdb46db36fc78d7b9af4.

In post-review discussion, William convinced me this was a bad
idea.  This allows a `const message_t` to be passed to sd-bus
functions which take a non-const `message*`, likely changing
the contents in a non-const way.

Change-Id: I6803b5dee80340587618dd9dc1804d96f9f6a0e2
Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
diff --git a/include/sdbusplus/message.hpp b/include/sdbusplus/message.hpp
index 4824997..b049a8f 100644
--- a/include/sdbusplus/message.hpp
+++ b/include/sdbusplus/message.hpp
@@ -485,7 +485,7 @@
      * This api should be used sparingly and carefully, as it opens a number of
      * possibilities for race conditions, RAII destruction issues, and runtime
      * problems when using the sd-bus c api.  Here be dragons. */
-    msgp_t get() const
+    msgp_t get()
     {
         return _msg.get();
     }