msvc: Fix a pointer arithmetic compilation error
In particular, msvc issues the following error:
src/tree.c(101) : error C2036: 'void *' : unknown size
This is caused by attempting to perform arithmetic with a void pointer.
Although using pointer arithmetic on void pointers is supported by gcc
as a language extension, it is not legal in standard C. In order to
suppress the error, we cast the pointer to (char *) so that the compiler
can determine the correct size of the data type.
Signed-off-by: Ramsay Jones <ramsay@ramsay1.demon.co.uk>