avoid GCC warning -Wignored-qualifiers
avoid GCC warning -Wignored-qualifiers

The warning was:
include/sajson.h:150:9: warning: type qualifiers ignored on function return type [-Wignored-qualifiers]
         const size_t length() const {
t089/sajson,chadaustin/sajson,t089/sajson,chadaustin/sajson,iboB/sajson,chadaustin/sajson,chadaustin/sajson,iboB/sajson,t089/sajson,t089/sajson,iboB/sajson,iboB/sajson,iboB/sajson,t089/sajson,chadaustin/sajson,iboB/sajson,chadaustin/sajson