[MIPS] Fix seccomp-bpf compile error for MIPS

sandbox/linux/seccomp-bpf-helpers/sigsys_handlers.cc

Index: sandbox/linux/seccomp-bpf-helpers/sigsys_handlers.cc
diff --git a/sandbox/linux/seccomp-bpf-helpers/sigsys_handlers.cc b/sandbox/linux/seccomp-bpf-helpers/sigsys_handlers.cc
index 85d3f70828c1e658d737bf0932f1d8e176a9eced..25398b6a48acb4a6b8689b9bc1bd2a1598e13c66 100644
--- a/sandbox/linux/seccomp-bpf-helpers/sigsys_handlers.cc
+++ b/sandbox/linux/seccomp-bpf-helpers/sigsys_handlers.cc
@@ -13,6 +13,11 @@
 #include "build/build_config.h"
 #include "sandbox/linux/seccomp-bpf/sandbox_bpf.h"
 
+#if defined(__mips__)
+// __NR_Linux, is defined in <asm/unistd.h>.
+#include <asm/unistd.h>
+#endif
+
 #define SECCOMP_MESSAGE_COMMON_CONTENT "seccomp-bpf failure"
 #define SECCOMP_MESSAGE_CLONE_CONTENT "clone() failure"
 #define SECCOMP_MESSAGE_PRCTL_CONTENT "prctl() failure"