[gnome-software] Fix build with RPM support disabled

[Robert Ancell <rancell src gnome org>]

commit 3cbc73da3029465f1c40ff826dc6b472145d9cb2
Author: Robert Ancell <robert ancell canonical com>
Date:   Fri Apr 21 14:15:15 2017 +1200

    Fix build with RPM support disabled

 plugins/ostree/meson.build |   27 ++++++++++++++-------------
 1 files changed, 14 insertions(+), 13 deletions(-)
---
diff --git a/plugins/ostree/meson.build b/plugins/ostree/meson.build
index 04c0805..8d5daea 100644
--- a/plugins/ostree/meson.build
+++ b/plugins/ostree/meson.build
@@ -13,16 +13,17 @@ shared_module(
   dependencies : [ plugin_libs, ostree ]
 )
 
-shared_module(
-  'gs_plugin_rpm',
-  sources : 'gs-plugin-rpm.c',
-  include_directories : [
-    include_directories('../..'),
-    include_directories('../../lib'),
-  ],
-  install : true,
-  install_dir: plugin_dir,
-  c_args : cargs,
-  dependencies : [ plugin_libs, rpm ]
-)
-
+if get_option('enable-rpm')
+  shared_module(
+    'gs_plugin_rpm',
+    sources : 'gs-plugin-rpm.c',
+    include_directories : [
+      include_directories('../..'),
+      include_directories('../../lib'),
+    ],
+    install : true,
+    install_dir: plugin_dir,
+    c_args : cargs,
+    dependencies : [ plugin_libs, rpm ]
+  )
+endif