map_serializers_unittest: fix unused variable warning

Building with clang triggers warnings (which for us breaks the build):
src/processor/map_serializers_unittest.cc:52:11: error: 
      unused variable 'kSizeOfInt' [-Werror,-Wunused-const-variable]
1 error generated.

Fix by Yunlian Jiang.

BUG=chromium:311720 (http://crbug.com/311720)
TEST=FEATURES="test" emerge-lumpy google-breakpad passes


git-svn-id: http://google-breakpad.googlecode.com/svn/trunk@1229 4c0a9323-5329-0410-9bdc-e9ce6186880e