meson: rename systemd dependency

Rename the `systemd` dependency to `systemd_dep` to follow meson
conventions.

Change-Id: Iab99e3d028fcc2df29fc53a9e95e2e08a1b265d1
Signed-off-by: Jagpal Singh Gill <paligill@gmail.com>
diff --git a/meson.build b/meson.build
index 39d4923..e8a2f4f 100644
--- a/meson.build
+++ b/meson.build
@@ -59,8 +59,8 @@
 
 ssl_dep = dependency('openssl')
 
-systemd = dependency('systemd')
-systemd_system_unit_dir = systemd.get_variable(
+systemd_dep = dependency('systemd')
+systemd_system_unit_dir = systemd_dep.get_variable(
     'systemd_system_unit_dir',
     pkgconfig_define: ['prefix', get_option('prefix')],
 )