libc: tag missing functions in system headers.
This matches recent changes in the NDK header.
We enclose missing functions in #if 0 .. #endif blocks
with a clear "MISSING" in comments in order to locate
them later.
Change-Id: I87b3a62e777897e75c9243360fb0a82bcc53d9fb
diff --git a/libc/include/signal.h b/libc/include/signal.h
index 7bc3145..4401164 100644
--- a/libc/include/signal.h
+++ b/libc/include/signal.h
@@ -120,6 +120,7 @@
extern int raise(int);
extern int kill(pid_t, int);
extern int killpg(int pgrp, int sig);
+extern int sigaltstack(const stack_t *ss, stack_t *oss);
__END_DECLS