ref-filter: move struct atom_value to ref-filter.c

Since atom_value is only required for the internal working of
ref-filter it doesn't belong in the public header.

Helped-by: Eric Sunshine <sunshine@sunshineco.com>
Mentored-by: Christian Couder <christian.couder@gmail.com>
Mentored-by: Matthieu Moy <matthieu.moy@grenoble-inp.fr>
Signed-off-by: Karthik Nayak <karthik.188@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

ref-filter.c

@@ -55,6 +55,11 @@ static struct {
 	{ "color" },
 };
 
+struct atom_value {
+	const char *s;
+	unsigned long ul; /* used for sorting when not FIELD_STR */
+};
+
 /*
  * An atom is a valid field atom listed above, possibly prefixed with
  * a "*" to denote deref_tag().

ref-filter.h

@@ -16,10 +16,7 @@
 #define FILTER_REFS_INCLUDE_BROKEN 0x1
 #define FILTER_REFS_ALL 0x2
 
-struct atom_value {
-	const char *s;
-	unsigned long ul; /* used for sorting when not FIELD_STR */
-};
+struct atom_value;
 
 struct ref_sorting {
 	struct ref_sorting *next;