MINOR: Configure the `cpp` userdiff driver for *.[ch] in .gitattributes

This might improve the output of `git diff` in certain cases. Especially
`git diff --word-diff` will be much more useful.

Does not affect the generated code, may be backported for consistency if
desired.
diff --git a/.gitattributes b/.gitattributes
index cd3d1ff..4b56645 100644
--- a/.gitattributes
+++ b/.gitattributes
@@ -1,2 +1,3 @@
 SUBVERS export-subst
 VERDATE export-subst
+*.[ch] diff=cpp