build: add dependency for subproject use

When phosphor-dbus-interfaces is used as a subproject, it
needs to expose a 'dependency' which can be used by the
consuming project to get at the libphosphor_dbus.  Add it
using the 'declare_dependency' call.

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: If8984d86704d2109b826e3c10512d01e90fd6bbc
diff --git a/meson.build b/meson.build
index 8dc550c..5873bf0 100644
--- a/meson.build
+++ b/meson.build
@@ -118,3 +118,9 @@
     version: meson.project_version(),
     description: 'Generated sdbusplus bindings for phosphor-dbus-interfaces'
 )
+
+phosphor_dbus_interfaces_dep = declare_dependency(
+    include_directories: include_directories('gen'),
+    link_with: libphosphor_dbus,
+    dependencies: sdbusplus_dep
+)