patch 8.1.0659: build failure without the sign feature

Problem:    Build failure without the sign feature.
Solution:   Put the sign struct declarations outside of the #ifdef.
diff --git a/src/structs.h b/src/structs.h
index ae1c12e..4ac98b1 100644
--- a/src/structs.h
+++ b/src/structs.h
@@ -731,8 +731,6 @@
 #define PT_FLAG_INS_START_INCL	1	// insert at start included in property
 #define PT_FLAG_INS_END_INCL	2	// insert at end included in property
 
-
-#if defined(FEAT_SIGNS) || defined(PROTO)
 // Sign group
 typedef struct signgroup_S
 {
@@ -741,10 +739,6 @@
     char_u	sg_name[1];		// sign group name
 } signgroup_T;
 
-// Macros to get the sign group structure from the group name
-#define SGN_KEY_OFF	offsetof(signgroup_T, sg_name)
-#define HI2SG(hi)	((signgroup_T *)((hi)->hi_key - SGN_KEY_OFF))
-
 typedef struct signlist signlist_T;
 
 struct signlist
@@ -758,6 +752,11 @@
     signlist_T  *prev;		/* previous entry -- for easy reordering */
 };
 
+#if defined(FEAT_SIGNS) || defined(PROTO)
+// Macros to get the sign group structure from the group name
+#define SGN_KEY_OFF	offsetof(signgroup_T, sg_name)
+#define HI2SG(hi)	((signgroup_T *)((hi)->hi_key - SGN_KEY_OFF))
+
 // Default sign priority for highlighting
 #define SIGN_DEF_PRIO	10