Actually set the header guard in "linux-syscalls.h".

Spotted while merging a MIPS change.

Change-Id: I36fb5a07d0bba0c117e9fe9733957bd37ca4b4c0
diff --git a/libc/include/sys/linux-syscalls.h b/libc/include/sys/linux-syscalls.h
index a60d3fa..de64c6b 100644
--- a/libc/include/sys/linux-syscalls.h
+++ b/libc/include/sys/linux-syscalls.h
@@ -1,5 +1,6 @@
 /* auto-generated by gensyscalls.py, do not touch */
 #ifndef _BIONIC_LINUX_SYSCALLS_H_
+#define _BIONIC_LINUX_SYSCALLS_H_
 
 #if !defined __ASM_ARM_UNISTD_H && !defined __ASM_I386_UNISTD_H && !defined __ASM_MIPS_UNISTD_H
 #if defined __arm__ && !defined __ARM_EABI__ && !defined __thumb__