tests/vendor: pass extra linker flags

on OpenBSD, botan library is third-party and live in /usr/local which isn't a path included by default in linker.

tests/vendor/Makefile

@@ -1,6 +1,13 @@
 ODIN=../../odin
+ODINFLAGS=
+
+OS=$(shell uname)
+
+ifeq ($(OS), OpenBSD)
+    ODINFLAGS:=$(ODINFLAGS) -extra-linker-flags:-L/usr/local/lib
+endif
 
 all: botan_test
 
 botan_test:
-	$(ODIN) run botan -out=botan_hash -o:speed -no-bounds-check
+	$(ODIN) run botan -out=botan_hash -o:speed -no-bounds-check $(ODINFLAGS)