[MIPS] Fix seccomp-bpf compile error for MIPS

This change fixes the seccomp-bpf compile error
introduced by change:
https://chromium.googlesource.com/chromium/src/+/ab2d46af89c657ab4fd01ab00de1ba2a6ad73f8d

BUG=408845

Review URL: https://codereview.chromium.org/569333003

Cr-Commit-Position: refs/heads/master@{#295291}

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"