meson: add missing pkgconfig requires

In the past we have added a "Requires: libsystemd" into our pkgconfig
file so that users of the libsdbusplus library do not also need to
directly use a -lsystemd.  There are header files we provide that make
inline calls to functions from libsystemd.  Add this under meson.

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: Id3bc326dfbf405f3b5300db05ed3fa2e507521ca
diff --git a/meson.build b/meson.build
index caf8c01..41d9984 100644
--- a/meson.build
+++ b/meson.build
@@ -49,5 +49,6 @@
     libraries: libsdbusplus,
     name: meson.project_name(),
     version: meson.project_version(),
+    requires: libsystemd_pkg,
     description: 'C++ bindings for sdbus',
 )