fix external dummy_lock symbol inadvertently introduced in sigaction

commit 9b14ad54 introduced this
namespace violation.

src/signal/sigaction.c

@@ -7,7 +7,7 @@
 #include "lock.h"
 #include "ksigaction.h"
 
-volatile int dummy_lock[1] = { 0 };
+static volatile int dummy_lock[1] = { 0 };
 
 extern hidden volatile int __abort_lock[1];