Fix write_buildconfig_header.py self-reference (#4999)

This path changed way back in 55d6a5f ("Move //gn/* to //* (#3070)").
Remove the directory portion.

build/chip/write_buildconfig_header.py

@@ -114,7 +114,7 @@ def GetOptions():
 
 def WriteHeader(options):
   with open(options.output, 'w') as output_file:
-    output_file.write("// Generated by gn/chip/write_buildconfig_header.py\n")
+    output_file.write("// Generated by write_buildconfig_header.py\n")
     if options.rulename:
       output_file.write('// From "' + options.rulename + '"\n')