add NT_ARM_SYSTEM_CALL to elf.h

new regset in linux v3.18 for ptrace.
diff --git a/include/elf.h b/include/elf.h
index e3edbcc..a24ea9b 100644
--- a/include/elf.h
+++ b/include/elf.h
@@ -566,6 +566,7 @@
 #define NT_ARM_TLS	0x401
 #define NT_ARM_HW_BREAK	0x402
 #define NT_ARM_HW_WATCH	0x403
+#define NT_ARM_SYSTEM_CALL	0x404
 #define NT_METAG_CBUF	0x500
 #define NT_METAG_RPIPE	0x501
 #define NT_METAG_TLS	0x502