Don't include malloc.h header on OpenBSD.

Index: 3rdparty/bgfx/examples/common/nanovg/fontstash.h
--- 3rdparty/bgfx/examples/common/nanovg/fontstash.h.orig
+++ 3rdparty/bgfx/examples/common/nanovg/fontstash.h
@@ -261,7 +261,6 @@ int fons__tt_getGlyphKernAdvance(FONSttFontImpl *font,
 static void* fons__tmpalloc(size_t size, void* up);
 static void fons__tmpfree(void* ptr, void* up);
 #else
-#	include <malloc.h>
 #	include <string.h>
 #endif // 0
 
