duo_unix: sandbox headers

duo_unix installs a pkgconfig file so moving the header directory should
be safe.

Closes Homebrew/homebrew#32510.

Formula/duo_unix.rb

@@ -4,6 +4,7 @@ class DuoUnix < Formula
   homepage "https://www.duosecurity.com/docs/duounix"
   url "https://dl.duosecurity.com/duo_unix-1.9.11.tar.gz"
   sha1 "4cb4e585b69fbc6a0a3635bc241fa22653c2f9c4"
+  revision 1
 
   depends_on "openssl"
 
@@ -12,6 +13,7 @@ class DuoUnix < Formula
                           "--disable-silent-rules",
                           "--prefix=#{prefix}",
                           "--sysconfdir=#{etc}",
+                          "--includedir=#{include}/duo",
                           "--with-openssl=#{Formula["openssl"].opt_prefix}"
     system "make", "install"
   end