CmakeLists.txt: Add missing flag

Flag is present in configure.ac, but wasn't added here.

Tested: Can build with Cmake

Change-Id: I7dc9e8994fdfced9863cff4bd246de89f475a3f2
Signed-off-by: James Feist <james.feist@linux.intel.com>
diff --git a/CMakeLists.txt b/CMakeLists.txt
index 5b57637..f3f05bd 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -3,6 +3,7 @@
 add_definitions (-DMCTP_LOG_STDERR)
 add_definitions (-DMCTP_HAVE_FILEIO)
 add_definitions (-DMCTP_HAVE_STDIO)
+add_definitions (-DMCTP_DEFAULT_ALLOC)
 
 add_library (libmctp STATIC alloc.c core.c log.c libmctp.h serial.c)