xzembed: add fallthrough annotations to fix build with GCC7

GCC7 is more strict than previous versions; add missing fall through
annotations. Style is modeled after similar annotations in xz_dec_lzma2.c.
diff --git a/linux/lib/xz/xz_dec_stream.c b/linux/lib/xz/xz_dec_stream.c
index d652550..a35d317 100644
--- a/linux/lib/xz/xz_dec_stream.c
+++ b/linux/lib/xz/xz_dec_stream.c
@@ -604,6 +604,8 @@
 			if (ret != XZ_OK)
 				return ret;
 
+		/* Fall through */
+
 		case SEQ_BLOCK_START:
 			/* We need one byte of input to continue. */
 			if (b->in_pos == b->in_size)
@@ -627,6 +629,8 @@
 			s->temp.pos = 0;
 			s->sequence = SEQ_BLOCK_HEADER;
 
+		/* Fall through */
+
 		case SEQ_BLOCK_HEADER:
 			if (!fill_temp(s, b))
 				return XZ_OK;
@@ -637,6 +641,8 @@
 
 			s->sequence = SEQ_BLOCK_UNCOMPRESS;
 
+		/* Fall through */
+
 		case SEQ_BLOCK_UNCOMPRESS:
 			ret = dec_block(s, b);
 			if (ret != XZ_STREAM_END)
@@ -664,6 +670,8 @@
 
 			s->sequence = SEQ_BLOCK_CHECK;
 
+		/* Fall through */
+
 		case SEQ_BLOCK_CHECK:
 			if (s->check_type == XZ_CHECK_CRC32) {
 				ret = crc_validate(s, b, 32);
@@ -713,6 +721,8 @@
 
 			s->sequence = SEQ_INDEX_CRC32;
 
+		/* Fall through */
+
 		case SEQ_INDEX_CRC32:
 			ret = crc_validate(s, b, 32);
 			if (ret != XZ_STREAM_END)
@@ -721,6 +731,8 @@
 			s->temp.size = STREAM_HEADER_SIZE;
 			s->sequence = SEQ_STREAM_FOOTER;
 
+		/* Fall through */
+
 		case SEQ_STREAM_FOOTER:
 			if (!fill_temp(s, b))
 				return XZ_OK;