Exclude LLVM files from whitespace checks

Commit 9044fc1d45 added some files from upstream LLVM.  These files
have different whitespace rules, which make the git whitespace checks
powered by gitattributes fail.  To fix, add those files to the exclude
list.

.gitattributes

@@ -25,9 +25,12 @@ src/interfaces/libpq/test/expected.out	whitespace=-blank-at-eof
 # These files are maintained or generated elsewhere.  We take them as is.
 configure				-whitespace
 ppport.h				-whitespace
+src/backend/jit/llvm/SectionMemoryManager.cpp		-whitespace
+src/backend/jit/llvm/SectionMemoryManager.LICENSE	-whitespace
 src/backend/regex/COPYRIGHT		-whitespace
 src/backend/regex/re_syntax.n		-whitespace
 src/backend/snowball/libstemmer/*.c	-whitespace
 src/backend/utils/mb/Unicode/*-std.txt	-whitespace
+src/include/jit/SectionMemoryManager.h	-whitespace
 src/include/snowball/libstemmer/*	-whitespace
 src/timezone/data/*			-whitespace