Update atom-helpers.h

Added _alignment_padding to _LV2_Atom_Buffer.

Fixes #1902.

from,

https://lv2plug.in/ns/ext/atom

"An LV2_Atom has a 32-bit size and type, followed by a body of size bytes. Atoms MUST be 64-bit aligned."

source/includes/lv2/atom-helpers.h

@@ -54,6 +54,7 @@ struct _LV2_Atom_Buffer
 	uint32_t capacity;
 	uint32_t chunk_type;
 	uint32_t sequence_type;
+	uint32_t _alignment_padding;
 	LV2_Atom_Sequence atoms;
 
 } LV2_Atom_Buffer;