Fix GRE Source Routing Header definition

The Source Routing Header had routing defined as a char* for a field
of variable size. Since that field was not being used in the code, I
removed the pointer and added a comment.

src/decode-gre.h

@@ -16,9 +16,11 @@
  */
 
 /**
- * \file
+ * \file decode-gre.h
  *
  * \author Breno Silva <breno.silva@gmail.com>
+ *
+ * Generic Route Encapsulation (GRE) from RFC 1701.
  */
 
 #ifndef __DECODE_GRE_H__
@@ -39,12 +41,14 @@ typedef struct GREHdr_
 
 } __attribute__((__packed__)) GREHdr;
 
+/* Generic Routing Encapsulation Source Route Entries (SREs).
+ * The header is followed by a variable amount of Routing Information.
+ */
 typedef struct GRESreHdr_
 {
     uint16_t af; /**< Address family */
     uint8_t sre_offset;
     uint8_t sre_length;
-    uint8_t *routing;
 } __attribute__((__packed__)) GRESreHdr;
 
 #define GRE_VERSION_0           0x0000