sd-lldp: move ETHERTYPE_LLDP to missing.h

After all, most ETHERTYPE variables are defined in the system headers, hence
define these where we defined all other fill-ins for system headers.

src/basic/missing.h

@@ -31,6 +31,7 @@
 #include <linux/neighbour.h>
 #include <linux/oom.h>
 #include <linux/rtnetlink.h>
+#include <net/ethernet.h>
 #include <stdlib.h>
 #include <sys/resource.h>
 #include <sys/syscall.h>
@@ -1168,4 +1169,8 @@ static inline key_serial_t request_key(const char *type, const char *description
 #define char16_t uint16_t
 #endif
 
+#ifndef ETHERTYPE_LLDP
+#define ETHERTYPE_LLDP 0x88cc
+#endif
+
 #endif

src/libsystemd-network/lldp.h

@@ -22,8 +22,6 @@
 
 #define LLDP_MULTICAST_ADDR     { 0x01, 0x80, 0xc2, 0x00, 0x00, 0x0e }
 
-#define ETHERTYPE_LLDP          0x88cc
-
 /* IEEE 802.3AB Clause 9: TLV Types */
 typedef enum LLDPTypes {
         LLDP_TYPE_END                  =   0,