Add GitLab CI (MR 7)

Closes https://gitlab.com/postmarketOS/openrc-settingsd/-/issues/4

.gitlab-ci.yml

+build:alpine-locale-style=default:
+  image: alpine:edge
+  before_script:
+    - apk add --no-cache build-base meson dbus-dev glib-dev libdaemon-dev openrc-dev polkit-dev
+  script:
+    - meson setup -Dlocale-style=default _build
+    - meson compile -C _build
+
+build:alpine-locale-style=gentoo:
+  image: alpine:edge
+  before_script:
+    - apk add --no-cache build-base meson dbus-dev glib-dev libdaemon-dev openrc-dev polkit-dev
+  script:
+    - meson setup -Dlocale-style=gentoo _build
+    - meson compile -C _build