util/build-id: define ElfW and NT_GNU_BUILD_ID if needed

Define ElfW() and NT_GNU_BUILD_ID if needed as these defines are not
present on at least OpenBSD and FreeBSD.  Fixes the build on OpenBSD.

Fixes: d4fa083e11 ("util: Add utility build-id code.")
Signed-off-by: Jonathan Gray <jsg@jsg.id.au>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>

src/util/build_id.c

@@ -28,6 +28,14 @@
 
 #include "build_id.h"
 
+#ifndef NT_GNU_BUILD_ID
+#define NT_GNU_BUILD_ID 3
+#endif
+
+#ifndef ElfW
+#define ElfW(type) Elf_##type
+#endif
+
 #define ALIGN(val, align)      (((val) + (align) - 1) & ~((align) - 1))
 
 struct build_id_note {