test: update issue5089.go for recent gccgo changes

As of https://golang.org/cl/43456 gccgo now gives a better error
message for this test.

Before:
    fixedbugs/issue5089.go:13:1: error: redefinition of ‘bufio.Buffered’: receiver name changed
     func (b *bufio.Reader) Buffered() int { // ERROR "non-local|redefinition"
     ^
    fixedbugs/issue5089.go:11:13: note: previous definition of ‘bufio.Buffered’ was here
     import "bufio" // GCCGO_ERROR "previous"
                 ^

Now:
    fixedbugs/issue5089.go:13:7: error: may not define methods on non-local type
     func (b *bufio.Reader) Buffered() int { // ERROR "non-local|redefinition"
           ^

Change-Id: I4112ca8d91336f6369f780c1d45b8915b5e8e235
Reviewed-on: https://go-review.googlesource.com/c/130955
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

test/fixedbugs/issue5089.go

@@ -8,7 +8,7 @@
 
 package p
 
-import "bufio"	// GCCGO_ERROR "previous"
+import "bufio"
 
 func (b *bufio.Reader) Buffered() int { // ERROR "non-local|redefinition"
 	return -1