debug/elf: fix offset for GNU version symbols

Since we no longer skip the first entry when reading a symbol table,
we no longer need to allow for the offset difference when processing
the GNU version symbols.

Unbreaks builds on Linux.

R=golang-dev, agl, iant
CC=golang-dev
https://golang.org/cl/6843057

src/pkg/debug/elf/file.go

@@ -700,8 +700,8 @@ func (f *File) gnuVersionInit(str []byte) {
 // gnuVersion adds Library and Version information to sym,
 // which came from offset i of the symbol table.
 func (f *File) gnuVersion(i int, sym *ImportedSymbol) {
-	// Each entry is two bytes; skip undef entry at beginning.
-	i = (i + 1) * 2
+	// Each entry is two bytes.
+	i = i * 2
 	if i >= len(f.gnuVersym) {
 		return
 	}