compiler: fix quoting of //go:linkname in error message

Change-Id: I2bb3fa67fac2b4fe32893cdad8f6e37261b11216
Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/195718
Reviewed-by: Than McIntosh <thanm@google.com>
Reviewed-by: Cherry Zhang <cherryyz@google.com>
1 file changed