plugins: fix meson.build

We create gst project from:

 - gst-project-maker
 - gst-element-maker

For this we need to fix the meson.build file base on our needs. To be
clear:

Since it is an element created by inheriting videofilter class, it is
necessary to add a dependency to meson.build.
Other thing is that function gst_element_register is duplicated, so
delete simplefilterplugin.c file and update plugin meson.build

Signed-off-by: Tommaso Merciai <tomm.merciai@gmail.com>

plugins/meson.build

@@ -1,8 +1,11 @@
 lib_args = common_args + []
 
+gstvideo_dep = dependency('gstreamer-video-1.0',
+  fallback: ['gst-plugins-base', 'video_dep'])
+
+
 # sources used to compile this plug-in
 plugin_sources = [
-  'gstsimplefilterplugin.c',
   'gstsimplefilter.c',
   'gstsimplefilter.h'
 ]
@@ -11,7 +14,7 @@ shlib = shared_library('gstsimplefilter',
   plugin_sources,
   c_args : lib_args,
   include_directories: [configinc],
-  dependencies : plugin_deps,
+  dependencies : [plugin_deps, gstvideo_dep],
   gnu_symbol_visibility : 'hidden',
   install : true,
   install_dir : plugins_install_dir,