add uapi guards for new netinet/ip.h conflict with struct iphdr

This ensures that the musl definition of 'struct iphdr' does not conflict
with the Linux kernel UAPI definition of it.

Some software, i.e. net-tools, will not compile against 5.4 kernel headers
without this patch and the corresponding Linux kernel patch.

include/netinet/ip.h

@@ -190,6 +190,8 @@ struct ip_timestamp {
 
 #define	IP_MSS		576
 
+#define __UAPI_DEF_IPHDR	0
+
 #ifdef __cplusplus
 }
 #endif