Auto merge of #1568 - ltratt:master, r=gnzlbg

Add two missing pthread calls to the OpenBSD support.

These are shamelessly copied from the NetBSD support, whose prototypes in this regard are identical to OpenBSD's.

src/unix/bsd/netbsdlike/openbsd/mod.rs

@@ -1446,6 +1446,15 @@ extern "C" {
         len: ::size_t,
         prot: ::c_int,
     ) -> ::c_int;
+    pub fn pthread_attr_getguardsize(
+        attr: *const ::pthread_attr_t,
+        guardsize: *mut ::size_t,
+    ) -> ::c_int;
+    pub fn pthread_attr_getstack(
+        attr: *const ::pthread_attr_t,
+        stackaddr: *mut *mut ::c_void,
+        stacksize: *mut ::size_t,
+    ) -> ::c_int;
     pub fn pthread_main_np() -> ::c_int;
     pub fn pthread_set_name_np(tid: ::pthread_t, name: *const ::c_char);
     pub fn pthread_stackseg_np(