std::rand: fix dragonflybsd after rust-lang#121942.

Author: devnexen

library/std/src/sys/pal/unix/rand.rs

@@ -66,13 +66,20 @@ mod imp {
         target_os = "espidf",
         target_os = "horizon",
         target_os = "freebsd",
-        target_os = "dragonfly",
         netbsd10
     ))]
     fn getrandom(buf: &mut [u8]) -> libc::ssize_t {
         unsafe { libc::getrandom(buf.as_mut_ptr().cast(), buf.len(), 0) }
     }
 
+    #[cfg(target_os = "dragonfly")]
+    fn getrandom(buf: &mut [u8]) -> libc::ssize_t {
+        extern "C" {
+            fn getrandom(buf: *mut c_void, buflen: size_t, flags: c_uint) -> c_int;
+        }
+        unsafe { getrandom(buf.as_mut_ptr().cast(), buf.len(), 0) }
+    }
+
     #[cfg(not(any(
         target_os = "linux",
         target_os = "android",