Don't use malloc.h header on OpenBSD.

Index: 3rdparty/bx/src/allocator.cpp
--- 3rdparty/bx/src/allocator.cpp.orig
+++ 3rdparty/bx/src/allocator.cpp
@@ -5,8 +5,6 @@
 
 #include <bx/allocator.h>
 
-#include <malloc.h>
-
 #ifndef BX_CONFIG_ALLOCATOR_NATURAL_ALIGNMENT
 #	define BX_CONFIG_ALLOCATOR_NATURAL_ALIGNMENT 8
 #endif // BX_CONFIG_ALLOCATOR_NATURAL_ALIGNMENT
