fix the CMake build

Summary:
D19222635 added a `ReentrantAllocatorTest.cpp` test file but in the
CMakeLists.txt file it listed `MmapAllocatorTest.cpp` instead.

Reviewed By: yfeldblum

Differential Revision: D19650216

fbshipit-source-id: 794ced255de6781a05da7114eb7f8b3258f90a8c

CMakeLists.txt

@@ -749,7 +749,7 @@ if (BUILD_TESTS)
 
     DIRECTORY memory/test/
       TEST arena_test SOURCES ArenaTest.cpp
-      TEST mmap_allocator_test SOURCES MmapAllocatorTest.cpp
+      TEST reentrant_allocator_test SOURCES ReentrantAllocatorTest.cpp
       TEST thread_cached_arena_test WINDOWS_DISABLED
         SOURCES ThreadCachedArenaTest.cpp
       TEST mallctl_helper_test SOURCES MallctlHelperTest.cpp