gitattributes: Ignore time zone data files for whitespace checks

The latest update introduced some funny whitespace, but since they are
externally maintained files, we add them to the list of files to ignore.

Author: petere

.gitattributes

@@ -33,3 +33,4 @@ src/backend/regex/re_syntax.n		-whitespace
 src/backend/snowball/libstemmer/*.c	-whitespace
 src/backend/utils/mb/Unicode/*-std.txt	-whitespace
 src/include/snowball/libstemmer/*	-whitespace
+src/timezone/data/*			-whitespace