vde_l3: Add static to inline functions

Add static to inline functions to avoid the following build failure
with BR2_OPTIMIZE_0:

/home/autobuild/autobuild/instance-11/output-1/host/lib/gcc/mips64-buildroot-linux-uclibc/10.3.0/../../../../mips64-buildroot-linux-uclibc/bin/ld: vde_l3.o: in function `ip_output_ready':
vde_l3.c:(.text+0xb30): undefined reference to `iphead'

Fixes:
 - http://autobuild.buildroot.org/results/5eeefcdac0b8974a424a885fdc2e82f7cb4b617d

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>

diff -Nura vde2-2.3.2.orig/src/vde_l3/vde_l3.c vde2-2.3.2/src/vde_l3/vde_l3.c
--- vde2-2.3.2.orig/src/vde_l3/vde_l3.c	2022-04-21 11:05:17.393452698 +0200
+++ vde2-2.3.2/src/vde_l3/vde_l3.c	2022-04-21 11:10:36.509804404 +0200
@@ -100,17 +100,17 @@
 };
 
 
-inline struct vde_ethernet_header *ethhead(struct vde_buff *vdb)
+static inline struct vde_ethernet_header *ethhead(struct vde_buff *vdb)
 {
 	return (struct vde_ethernet_header*)(vdb->data);
 }
 
-inline struct iphdr *iphead(struct vde_buff *vdb)
+static inline struct iphdr *iphead(struct vde_buff *vdb)
 {
 	return (struct iphdr*)(vdb->data + 14);
 }
 
-inline void *payload(struct vde_buff *vdb)
+static inline void *payload(struct vde_buff *vdb)
 {
 	return (uint8_t*)(vdb->data + 14 + sizeof(struct iphdr));
 }