MINOR: checks: Export the tcpcheck_eval_ret enum

This enum will be used to define custom function for tcp-check expect rules.
diff --git a/include/types/checks.h b/include/types/checks.h
index c44fe8e..2205633 100644
--- a/include/types/checks.h
+++ b/include/types/checks.h
@@ -246,6 +246,12 @@
 	};
 };
 
+enum tcpcheck_eval_ret {
+	TCPCHK_EVAL_WAIT = 0,
+	TCPCHK_EVAL_STOP,
+	TCPCHK_EVAL_CONTINUE,
+};
+
 enum tcpcheck_expect_type {
 	TCPCHK_EXPECT_UNDEF = 0, /* Match is not used. */
 	TCPCHK_EXPECT_STRING, /* Matches a string. */