[klibc] include: [sys/elfcommon.h] define STN_UNDEF

Noticed while trying to compile kexec against klibc.

Signed-off-by: maximilian attems <max@stro.at>
diff --git a/usr/include/sys/elfcommon.h b/usr/include/sys/elfcommon.h
index ad5e459..300ff4e 100644
--- a/usr/include/sys/elfcommon.h
+++ b/usr/include/sys/elfcommon.h
@@ -147,6 +147,9 @@
 #define SHN_COMMON      0xfff2
 #define SHN_HIRESERVE   0xffff
 
+/* End of a chain.  */
+#define STN_UNDEF	0
+
 /* Lenght of magic at the start of a file */
 #define EI_NIDENT	16