BUILD: check: use __fallthrough in __health_adjust()

This avoids one build warning when preprocessing happens before compiling
with gcc >= 7.
diff --git a/src/check.c b/src/check.c
index 722292c..e548b17 100644
--- a/src/check.c
+++ b/src/check.c
@@ -686,7 +686,7 @@
 			if (s->check.health > s->check.rise)
 				s->check.health = s->check.rise + 1;
 
-			/* fall through */
+			__fallthrough;
 
 		case HANA_ONERR_FAILCHK:
 		/* simulate a failed health check */