Hide `sdshdr' structure with flexible array member.

[lemzwerg]

The idea is that sds.c gets compiled with a C99 compliant compiler, and a C++ program then uses it. To do so, it needs access to sds.h, and this commit makes the header file valid C++ code.

Note that this patch omits the extern "C" guard necessary for C++, since it is already available in other push requests (https://github.com/antirez/sds/pull/31 and https://github.com/antirez/sds/pull/32).