Do not return const from postfix increment operator.

Turns out this doesn't work on Windows and the advice in
https://www.securecoding.cert.org/confluence/display/cplusplus/DCL21-CPP.+Overloaded+postfix+increment+and+decrement+operators+should+return+a+const+object
this change was based probably isn't all that great.
Reverses part of f5eda16.

Author: joto

include/osmium/io/output_iterator.hpp

@@ -105,7 +105,7 @@ namespace osmium {
                 return *this;
             }
 
-            const OutputIterator operator++(int) noexcept {
+            OutputIterator operator++(int) noexcept {
                 return *this;
             }