change sign for `blksize_t` type under openbsd

src/unix/bsd/openbsdlike/openbsd.rs

@@ -2,7 +2,7 @@ pub type clock_t = i64;
 pub type suseconds_t = i64;
 pub type dev_t = i32;
 pub type sigset_t = ::c_uint;
-pub type blksize_t = ::uint32_t;
+pub type blksize_t = ::int32_t;
 pub type fsblkcnt_t = ::uint64_t;
 pub type fsfilcnt_t = ::uint64_t;
 pub type pthread_attr_t = *mut ::c_void;