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>

go/gogo.cc

diff --git a/go/gogo.cc b/go/gogo.cc
index 3ff88cb..e7af673 100644
--- a/go/gogo.cc
+++ b/go/gogo.cc
@@ -2541,7 +2541,7 @@
     {
       if (ext_name.empty())
 	go_error_at(loc,
-		    ("//%<go:linkname%> missing external name "
+		    ("%<//go:linkname%> missing external name "
 		     "for declaration of %s"),
 		    go_name.c_str());
       else