patch 9.0.0594: Makefile error message causes a shell error

Problem:    Makefile error message causes a shell error.
Solution:   Put the message in single quotes. (closes #11232)
diff --git a/src/version.c b/src/version.c
index 8f503f2..6b05454 100644
--- a/src/version.c
+++ b/src/version.c
@@ -700,6 +700,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    594,
+/**/
     593,
 /**/
     592,