SongSticker, playlist/soundcloud: add missing stdlib.h include

For free().

src/playlist/plugins/SoundCloudPlaylistPlugin.cxx

@@ -35,6 +35,7 @@
 #include <string>
 
 #include <string.h>
+#include <stdlib.h>
 
 static struct {
 	std::string apikey;

src/sticker/SongSticker.cxx

@@ -27,6 +27,7 @@
 
 #include <assert.h>
 #include <string.h>
+#include <stdlib.h>
 
 std::string
 sticker_song_get_value(const LightSong &song, const char *name)