[PellesC] fix _lseeki64() macro

[gvanem]

Compilng with latest version 10.0 of PellesC gives this error:

tool_cb_see.c(104): error #1050: Redefinition of macro '_lseeki64'.

It's <io.h> defines _lseeki64() as:

#define _lseeki64(h,o,m)  _lseek64(h,o,m)

PellesC' pre-processor seems to have an issue with redefining arguments inside the macro; using (h,o,m)
also works in curl. But a bit more elegant IMHO just to #undef and redefine.