vendor: botan: add OpenBSD support

vendor/botan/bindings/botan.odin

@@ -146,6 +146,8 @@ when ODIN_OS == .Windows {
     foreign import botan_lib "system:botan-2"
 } else when ODIN_OS == .Darwin {
     foreign import botan_lib "system:botan-2"
+} else when ODIN_OS == .OpenBSD {
+    foreign import botan_lib "system:botan-2"
 }
 
 @(default_calling_convention="c")
@@ -471,4 +473,4 @@ foreign botan_lib {
     fpe_destroy                         :: proc(fpe: fpe_t) -> c.int ---
     fpe_encrypt                         :: proc(fpe: fpe_t, x: mp_t, tweak: ^c.char, tweak_len: c.size_t) -> c.int ---
     fpe_decrypt                         :: proc(fpe: fpe_t, x: mp_t, tweak: ^c.char, tweak_len: c.size_t) -> c.int ---
-}
+}