Replacing the void * cast to the real type cast

[lmatthieu]

Replacing the void * cast to the real type cast to avoid compilation error on MacOSX.

On MacOSX this error is raised by the clang compiler: rmutil/sds.h:110:13: error: cannot initialize a variable of type 'struct sdshdr8 *' with an rvalue of type 'void *' SDS_HDR_VAR(8,s);

This update replace the void * cast to the real type cast.