build: Help attempts to syntax highlight Config.mk

Some attempts to syntax highlight Config.mk end up thinking that most of
Config.mk is a string, due to the unbalanced squote.  Provide a balancing
squote in a comment to compensate.

No functional change.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: George Dunlap <george.dunlap@citrix.com>

Config.mk

@@ -7,6 +7,7 @@ endif
 # Convenient variables
 comma   := ,
 squote  := '
+#' Balancing squote, to help syntax highlighting
 empty   :=
 space   := $(empty) $(empty)