patch 9.0.0606: system() opens a terminal window when "!" is in 'guioptions'

Problem:    system() opens a terminal window when using the GUI and "!" is in
            'guioptions'.
Solution:   Do not use a terminal window when the SHELL_SILENT flag is used.
            (closes #11202)
diff --git a/src/version.c b/src/version.c
index c9dab4d..5a4cb9f 100644
--- a/src/version.c
+++ b/src/version.c
@@ -700,6 +700,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    606,
+/**/
     605,
 /**/
     604,