BugFix: Syntax error in gen.py file

The correct form is `self._platform`, not `self_.platform`.

Change-Id: I94c4d75cf1db073a9167ea4b09da878658ba6bda
Reviewed-on: https://gn-review.googlesource.com/c/gn/+/17520
Reviewed-by: Takuto Ikuta <tikuta@google.com>
Reviewed-by: David Turner <digit@google.com>
Commit-Queue: David Turner <digit@google.com>

build/gen.py

diff --git a/build/gen.py b/build/gen.py
index 6f3652e..d1c649c 100755
--- a/build/gen.py
+++ b/build/gen.py
@@ -101,7 +101,7 @@
     return self._platform == 'zos'
 
   def is_serenity(self):
-    return self_.platform == 'serenity'
+    return self._platform == 'serenity'
 
 class ArgumentsList:
   """Helper class to accumulate ArgumentParser argument definitions