Replace the malloc.h header with stdlib.h.

Index: src/image/tim_memory_crt.cpp
--- src/image/tim_memory_crt.cpp.orig
+++ src/image/tim_memory_crt.cpp
@@ -23,7 +23,7 @@ Foundation, Inc., 59 Temple Place, Suite 330, Boston, 
  * CRT memory driver.
  */
 
-#if defined(__FreeBSD__) || (defined(__APPLE__) && defined(__MACH__))
+#if defined(__FreeBSD__) || defined(__OpenBSD__) || (defined(__APPLE__) && defined(__MACH__))
 #include <stdlib.h>
 #else
 #include <malloc.h>
