Fix for:
src/backend.c:613:6: warning: implicit declaration of function 'hid_init'
is invalid in C99
On ld.bfd arches, this led to undefined references at build time.

It appears that comms/libhidapi redefines hid_init(3) to hidapi_hid_init().

Index: src/backend.c
--- src/backend.c.orig
+++ src/backend.c
@@ -23,6 +23,9 @@
 #ifdef _WIN32
 #include <winsock2.h>
 #endif
+#ifdef __OpenBSD__
+#define hid_init hidapi_hid_init
+#endif
 #include <libsigrok/libsigrok.h>
 #include "libsigrok-internal.h"