meson: add dependency for libmapper

In order to allow 'libmapper' to be picked up as a meson subproject,
we need to declare the dependency.  Enabling this allows a trivial
wrap file such as the following to allow `dependency('mapper')` to
resolve in a wrap-build:

```
$ cat subprojects/phosphor-objmgr.wrap
[wrap-git]
url = https://github.com/openbmc/phosphor-objmgr
revision = HEAD

[provide]
mapper = mapper_dep
```

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: I45d8d3f69fd284bfe727e1b8e94fb42a6158a1b5
diff --git a/meson.build b/meson.build
index 0510bad..e86c378 100644
--- a/meson.build
+++ b/meson.build
@@ -56,6 +56,12 @@
     version: meson.project_version(),
     install: true)
 
+mapper_dep = declare_dependency(
+    link_with: libmapper,
+    include_directories: include_directories('libmapper'),
+    dependencies: [ dependency('libsystemd') ],
+)
+
 import('pkgconfig').generate(
     name: 'libmapper',
     description: 'OpenBMC service discovery utility library',