decode/erspan: move header struct to source file

ErspanHdr is not used outside of the decode-erspan.c compilation
unit, so define it there instead of in the header.

Ticket: #8329

src/decode-erspan.c

@@ -41,6 +41,12 @@
 #include "util-debug.h"
 #include "conf.h"
 
+typedef struct ErspanHdr_ {
+    uint16_t ver_vlan;
+    uint16_t flags_spanid;
+    uint32_t padding;
+} __attribute__((__packed__)) ErspanHdr;
+
 /**
  * \brief Functions to decode ERSPAN Type I and II packets
  */

src/decode-erspan.h

@@ -25,11 +25,6 @@
 #ifndef SURICATA_DECODE_ERSPAN_H
 #define SURICATA_DECODE_ERSPAN_H
 
-typedef struct ErspanHdr_ {
-    uint16_t ver_vlan;
-    uint16_t flags_spanid;
-    uint32_t padding;
-} __attribute__((__packed__)) ErspanHdr;
-
 void DecodeERSPANConfig(void);
+
 #endif /* SURICATA_DECODE_ERSPAN_H */