Update .ignore to exclude log files. (#14064)

Ignore custom-access.log and its rotated files.
diff --git a/.gitignore b/.gitignore
index 0dffb48..b504f33 100644
--- a/.gitignore
+++ b/.gitignore
@@ -50,3 +50,6 @@
 
 .git.exec.error
 
+# log files generated by testcase.
+dubbo-rpc/dubbo-rpc-api/custom-access.log*
+