libarchive: Set .gitattributes to allow trailing whitespace

We do not care about trailing whitespace in third-party code.

Utilities/cmlibarchive/.gitattributes

@@ -1,2 +1,2 @@
-*.h              whitespace=indent-with-non-tab
-*.c              whitespace=indent-with-non-tab
+*.h              whitespace=indent-with-non-tab,-blank-at-eol
+*.c              whitespace=indent-with-non-tab,-blank-at-eol