TT#175501 ngcp-emergency-mode: Import JSON module

We are using constants from the JSON module, so we need to import it.
Otherwise we get errors due to «use strict».

Change-Id: I07814e5100e40f5a0dbdf24d5f7992b4bf0934d3
(cherry picked from commit 1a62b5cc52)
(cherry picked from commit 85bf8f8988)

tools_bin/ngcp-emergency-mode

@@ -35,6 +35,7 @@ use URI;
 use NGCP::Panel::Utils::XMLDispatcher;
 use NGCP::API::Client;
 use Config::Simple;
+use JSON;
 use TryCatch;
 use Sys::Syslog qw(:standard :macros);
 use IO::Prompt::Tiny qw(prompt);