Reduce warnings/errors in libpayload when using picky compiler options

The new build system uses quite a few more -W flags for the compiler by
default than the old one. And that's for the better.

Change-Id: Ia8e3d28fb35c56760c2bd0983046c7067e8c5dd6
Signed-off-by: Patrick Georgi <patrick@georgi-clan.de>
Reviewed-on: http://review.coreboot.org/72
Tested-by: build bot (Jenkins)
Reviewed-by: Uwe Hermann <uwe@hermann-uwe.de>

payloads/libpayload/libc/sysinfo.c

@@ -30,8 +30,6 @@
 #include <libpayload.h>
 #include <sysinfo.h>
 
-extern struct sysinfo_t lib_sysinfo;
-
 int sysinfo_have_multiboot(unsigned long *addr)
 {
 	*addr = (unsigned long) lib_sysinfo.mbtable;