Don't scan libfoo_error.c for error codes.

This will have the effect that all dead error codes are removed
from given lib when make_errors.go runs with --reset flag.

Change-Id: I6303721c5d7cd18af7d47c95fdf3702a7628ad5a
Reviewed-on: https://boringssl-review.googlesource.com/1570
Reviewed-by: Adam Langley <agl@google.com>

util/make_errors.go

diff --git a/util/make_errors.go b/util/make_errors.go
index f770e3d..5fd75e2 100644
--- a/util/make_errors.go
+++ b/util/make_errors.go
@@ -90,7 +90,7 @@
 	}
 
 	for _, name := range filenames {
-		if !strings.HasSuffix(name, ".c") {
+		if !strings.HasSuffix(name, ".c") || name == sourcePath {
 			continue
 		}