libc: add sys/signal.h for compatibility

Some applications look for sys/signal.h instead of signal.h.
Work around those apps.

Change-Id: I76ac7744ebc56d196b5f0cb9ed381d32817436b9
diff --git a/libc/include/sys/signal.h b/libc/include/sys/signal.h
new file mode 100644
index 0000000..2e602da
--- /dev/null
+++ b/libc/include/sys/signal.h
@@ -0,0 +1 @@
+#include <signal.h>