After upx-3.96, the de-compressing stub for ELF moved

... so unpack() of newly-compressed ELF by upx-3.96 now says
"CantUnpackException: need a newer version of UPX"
        https://github.com/upx/upx/issues/595
	modified:   p_lx_elf.h
	modified:   packer.h

src/packer.h

@@ -116,6 +116,8 @@ public:
     virtual ~Packer();
     virtual void assertPacker() const;
 
+    // getVersion() enables detecting forward incompatibility of unpack()
+    // by old upx when newer upx changes the format of compressed output.
     virtual int getVersion() const = 0;
     // A unique integer ID for this executable format. See conf.h.
     virtual int getFormat() const = 0;