Don't use malloc.h header on OpenBSD.

Index: intern/guardedalloc/intern/mallocn_intern.h
--- intern/guardedalloc/intern/mallocn_intern.h.orig
+++ intern/guardedalloc/intern/mallocn_intern.h
@@ -39,6 +39,8 @@
 #  define HAVE_MALLOC_STATS
 #elif defined(__FreeBSD__)
 #  include <malloc_np.h>
+#elif defined(__OpenBSD__)
+/* nothing here */
 #elif defined(__APPLE__)
 #  include <malloc/malloc.h>
 #  define malloc_usable_size malloc_size
@@ -66,7 +68,8 @@ size_t malloc_usable_size(void *ptr);
 #  define UNLIKELY(x) (x)
 #endif
 
-#if !defined(__APPLE__) && !defined(__FreeBSD__) && !defined(__NetBSD__)
+#if !defined(__APPLE__) && !defined(__FreeBSD__) && !defined(__NetBSD__) && \
+	!defined(__OpenBSD__)
 // Needed for memalign on Linux and _aligned_alloc on Windows.
 
 #  include <malloc.h>
