refactor: adjust definition of off_t in wasi

The definition for the analogous `time_t` was using the exact same C
type as that declared in the "bottom half" of the WASI upstream C
library implementation.

On the other hand, the `off_t` type was explicitly declaring the type as
a signed 64-bit integer, which is equivalent in the case of WASI to the
upstream definition as a `c_longlong`. Still, this patch attempts to
reflect a mirrored definition, rather than an equivalent definition.

Author: dybucc

src/wasi/mod.rs

@@ -14,7 +14,7 @@ pub type ssize_t = isize;
 pub type ptrdiff_t = isize;
 pub type intptr_t = isize;
 pub type uintptr_t = usize;
-pub type off_t = i64;
+pub type off_t = c_longlong;
 pub type pid_t = i32;
 pub type clock_t = c_longlong;
 pub type time_t = c_longlong;