Fix -Wimplicit-fallthrough warnings

Add "falls through" comments to quench implicit-fallthrough warnings
which are enabled by -Wextra under GCC 7.
diff --git a/catalog.c b/catalog.c
index 6dfdfbb..f6186a1 100644
--- a/catalog.c
+++ b/catalog.c
@@ -2396,6 +2396,7 @@
 		case SGML_CATA_ENTITY:
 		    if (*cur == '%')
 			type = SGML_CATA_PENTITY;
+                    /* Falls through. */
 		case SGML_CATA_PENTITY:
 		case SGML_CATA_DOCTYPE:
 		case SGML_CATA_LINKTYPE: