Replace build.rs with link attributes (rust-random#205)

Author: newpavlov

build.rs

@@ -11,6 +11,7 @@ use core::{ffi::c_void, num::NonZeroU32, ptr};
 
 const BCRYPT_USE_SYSTEM_PREFERRED_RNG: u32 = 0x00000002;
 
+#[link(name = "bcrypt")]
 extern "system" {
     fn BCryptGenRandom(
         hAlgorithm: *mut c_void,