Resolve the ambiguity of the strprintf::fmt call for size_t type definition.

Upstream commit 48a935e372eafeb253659e224554f71b181977a9.

Index: include/strprintf.h
--- include/strprintf.h.orig
+++ include/strprintf.h
@@ -55,7 +55,7 @@ std::string fmt(std::string_view format, const uint64_
 	return detail::fmt_impl(format, argument, std::forward<Args>(args)...);
 }
 
-#ifdef __APPLE__
+#if defined(__APPLE__) || defined(__OpenBSD__)
 template<typename... Args>
 std::string fmt(std::string_view format, const std::size_t argument,
 	Args&& ... args)
