Fix compilation warnings in the MS-Windows build.

Fix compilation warnings in the MS-Windows build.

 src/w32proc.c (sys_wait): Declare 'signame' 'const char *', to be
 consistent with the change in return value of 'safe_strsignal'.

 nt/preprep.c (RVA_TO_PTR): Cast the result to 'void *', to avoid
 compiler warnings when using the value.