fix(build_sys): install the intl PHP extension (#208)

Upstream made the `intl` PHP extension a requirement[1]. Update the build
script to install it.

1. In commit ccd90236dc64645f3889e4efb684493dde68c949 in LibreBooking:
LibreBooking/librebooking@ccd9023

Author: JohnVillalovos

bin/build_sys.sh

@@ -15,6 +15,7 @@ apt-get upgrade --yes
 apt-get install --yes --no-install-recommends \
   libjpeg-dev \
   libldap-dev \
+  libicu-dev \
   libpng-dev \
   libfreetype6-dev \
   unzip
@@ -46,7 +47,7 @@ cp "$PHP_INI_DIR/php.ini-production" "$PHP_INI_DIR/php.ini"
 touch /usr/local/etc/php/conf.d/librebooking.ini
 
 docker-php-ext-configure gd --with-jpeg --with-freetype
-docker-php-ext-install mysqli gd ldap
+docker-php-ext-install mysqli gd ldap intl
 pecl install timezonedb
 docker-php-ext-enable timezonedb