add additional uapi guards for Linux kernel header files

With Linux kernel 4.16 it will be possible to guard more parts of the
Linux header files from a libc. Make use of this in musl to guard all
the structures and other definitions from the Linux header files which
are also defined by the header files provided by musl. This will make
it possible to compile source files which include both the libc
headers and the kernel userspace headers.

This extends the definitions done in commit 04983f2272 ("make
netinet/in.h suppress clashing definitions from kernel headers")

include/netinet/if_ether.h

@@ -133,5 +133,6 @@ do { \
 	(enaddr)[5] = ((uint8_t *)ipaddr)[3]; \
 } while(0)
 
+#define __UAPI_DEF_ETHHDR       0
 
 #endif