gather/external_ip: set Host header

http://ifconfig.me replies with 404 when Host header isn't set.

modules/auxiliary/gather/external_ip.rb

@@ -29,12 +29,19 @@ def initialize
     register_options(
       [
         Opt::RHOST('ifconfig.me'),
+        OptString.new('VHOST', [false, "HTTP server hostname", 'ifconfig.me']),
         OptBool.new('REPORT_HOST', [false, 'Add the found IP to the database', false])
       ])
 end
 
   def run
     connect
+
+    # Use VHOST when set.
+    def rhost
+      datastore['VHOST'] || datastore['RHOST']
+    end
+
     res = send_request_cgi({'uri' => '/ip', 'method' => 'GET' })
 
     if res.nil?