audio_output: Modules.am: add amem audio memory output module

Build audio memory output module.

modules/audio_output/Modules.am

@@ -9,8 +9,10 @@ SOURCES_auhal = auhal.c
 SOURCES_jack = jack.c
 SOURCES_pulse = pulse.c
 SOURCES_audioqueue = audioqueue.c
+SOURCES_amem = amem.c
 
-libvlc_LTLIBRARIES += libaout_file_plugin.la
+libvlc_LTLIBRARIES += libaout_file_plugin.la \
+    libamem_plugin.la
 
 libalsa_plugin_la_SOURCES = alsa.c
 libalsa_plugin_la_CFLAGS = $(AM_CFLAGS) $(ALSA_CFLAGS)