MT#16683 Switch panel dependency from libgd-gd2-perl to libgd-perl

We do not have package libgd-gd2-perl in Jessie anymore,
it is available as libgd-perl nowadays.

New installation works because libgd-perl provides libgd-gd2-perl:
>> apt-cache show libgd-perl
> Package: libgd-perl
> Source: libgd-perl (2.53-1)
> Version: 2.53-1+b1
> Provides: libgd-gd2-noxpm-perl, libgd-gd2-perl

While PRO upgrade failed because "apt-get dist-upgrade" decided to
remove ngcp-panel as cannot resolve dependencies:

> The following packages will be REMOVED:
>   cpp-4.7 gcc-4.7 gcc-4.7-base libaudit0 libfile-spec-perl libgd-gd2-perl
>   libperl5.14 libsnmp15 ngcp-ngcp-pro ngcp-panel ngcp-services-pro

In the same time 'aptitude dist-upgrade' solves this problems well.

What is strange here 'apt-get dist-upgrade' in CE works well
with the same ngcp-panel 0_o , I suspect removing libsnmp15
is affected apt-get logic here somehow.

Change-Id: I4533a9a50b3bde1c23e4183f0eb5edaa0ff77a07

debian/control

@@ -45,7 +45,7 @@ Depends: gettext,
          libfile-slurp-unicode-perl,
          libfile-spec-perl (>= 3.4000~) | perl-base (>= 5.20.0-60~),
          libfile-type-perl,
-         libgd-gd2-perl,
+         libgd-perl,
          libgeography-countries-perl,
          libhash-merge-simple-perl,
          libhtml-formhandler-model-dbic-perl,