properly define SIGSTKSZ under openbsd

Author: semarie

src/unix/bsd/openbsdlike/openbsd.rs

@@ -201,7 +201,7 @@ pub const _SC_RTSIG_MAX : ::c_int = 66;
 pub const _SC_SIGQUEUE_MAX : ::c_int = 70;
 pub const _SC_TIMER_MAX : ::c_int = 93;
 
-pub const SIGSTKSZ: ::size_t = 131072;
+pub const SIGSTKSZ: ::size_t = 40960;
 
 pub const FD_SETSIZE: usize = 1024;