gh-35027: gitignore another (temporary) autoconf file
    
This shows up on my system when configuring (but only temporarily),
nevertheless it is helpful to ignore it in case we accidentally `git add
*` at precisely the wrong moment without noticing

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URL: #35027
Reported by: Alex J Best
Reviewer(s): roed314

Author: Release Manager

.gitignore

@@ -19,6 +19,7 @@
 /config.status
 /configure
 /conftest*
+/confdefs.h
 
 /m4/sage_spkg_configures.m4