Fix an error for Centos 5

Unfortunately this doesn't get rust to compile on Centos 5.
Centos 5 uses glibc 2.5, but the snapshot-0 needs glibc 2.6+.

Author: erickt

src/rt/rust_signal.h

@@ -5,6 +5,7 @@
 class rust_signal {
 public:
     virtual void signal() = 0;
+    virtual ~rust_signal() {}
 };
 
 #endif /* RUST_SIGNAL_H */