Fix compilation on OpenBSD

Non-POSIX functions (like vasprintf() and swap16()) require
_BSD_SOURCE to be defined.

Signed-off-by: Benjamin Stürz <benni@stuerz.xyz>

zbuild.h

@@ -4,6 +4,9 @@
 #ifndef _ISOC11_SOURCE
 #  define _ISOC11_SOURCE 1 /* aligned_alloc */
 #endif
+#ifdef __OpenBSD__
+#  define _BSD_SOURCE 1
+#endif
 
 /* This has to be first include that defines any types */
 #if defined(_MSC_VER)