Link dropbear against libmagenta so that it can get to the crypto
syscalls.

Change-Id: I0be901067c819fbac865d2437ca25a51cea3f337
diff --git a/BUILD.gn b/BUILD.gn
index bb40350..b66085e 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -559,6 +559,8 @@
     "ENCRYPT_ONLY",
   ]
 
+  libs = [ "magenta" ]
+
   deps = [
     ":libtomcrypt",
     ":libtommath",