Meson 'ctpp2_include_path' has to be declared

[kelson42]

Otherwise you might get this kind of error:

$ rm -rf build/ && meson . build
The Meson build system
Version: 0.40.1
Source dir: /home/kiwix/kiwix-lib
Build dir: /home/kiwix/kiwix-lib/build
Build type: native build
Project name: kiwixlib
Native cpp compiler: c++ (gcc 5.4.0)
Build machine cpu family: x86_64
Build machine cpu: x86_64
Dependency threads found: YES
Found pkg-config: /usr/bin/pkg-config (0.29.1)
Native dependency icu-i18n found: YES 55.1
Native dependency libzim found: YES 1.4
Native dependency pugixml found: YES 1.8
Has header "ctpp2/CTPP2Logger.hpp": YES
Library ctpp2 found: YES
Program compile_resources.py found: YES (/home/kiwix/kiwix-lib/scripts/compile_resources.py)
Configuring kiwix_config.h using configuration

Meson encountered an error in file meson.build, line 86, column 5:
Unknown variable "ctpp2_include_path".

[mgautierfr]

Fixes #44