meson: Use .dylib suffix if darwin

For Mac OS, GST_EXTRA_MODULE_SUFFIX should be set as '.dylib'.

https://bugzilla.gnome.org/show_bug.cgi?id=793585
diff --git a/meson.build b/meson.build
index 21502ad..1d355c0 100644
--- a/meson.build
+++ b/meson.build
@@ -294,6 +294,10 @@
   cdata.set_quoted('DEFAULT_VIDEOSRC', 'videotestsrc')
 endif
 
+if host_system == 'darwin'
+  cdata.set_quoted('GST_EXTRA_MODULE_SUFFIX', '.dylib')
+endif
+
 # Mandatory GST deps
 gst_dep = dependency('gstreamer-1.0', version : gst_req,
   fallback : ['gstreamer', 'gst_dep'])