Disable macro redefined warnings on some includes.

The new kernel headers do:

  #define __bitwise __bitwise__

However, the code redefines __bitwise without undef'ing it first.

The files affected are the include files from external/e2fsprogs,
so the warnings will only be disabled around those includes.

This is a temporary fix, b/35721782 filed to fix this.

Test: Builds without warningers/errors.
Change-Id: I2eae12558c23be9bb3bf3931467b32eee5d57720
diff --git a/payload_generator/ext2_filesystem.cc b/payload_generator/ext2_filesystem.cc
index b452b41..ee2f8c2 100644
--- a/payload_generator/ext2_filesystem.cc
+++ b/payload_generator/ext2_filesystem.cc
@@ -17,8 +17,12 @@
 #include "update_engine/payload_generator/ext2_filesystem.h"
 
 #include <et/com_err.h>
+// TODO: Remove these pragmas when b/35721782 is fixed.
+#pragma clang diagnostic push
+#pragma clang diagnostic ignored "-Wmacro-redefined"
 #include <ext2fs/ext2_io.h>
 #include <ext2fs/ext2fs.h>
+#pragma clang diagnostic pop
 
 #include <map>
 #include <set>