common: Include <fmt/format.h> for fmt::format in string.hpp

string.hpp calls fmt::format and fmt::vformat but only included
<fmt/core.h>. As of fmt 12, <fmt/core.h> was reduced to a shim and the
format/vformat entry points moved to <fmt/format.h>, so building
against a system fmt 12.x fails with "no member named 'format' in
namespace 'fmt'" in every translation unit that pulls in string.hpp.

Include <fmt/format.h> instead. It is a superset of the old core.h, so
this is backward compatible with older fmt releases while restoring the
build on fmt 12.

Signed-off-by: Razvan Cojocaru <razvan.cojocaru@openvpn.com>

Author: rzvncj

openvpn/common/string.hpp

@@ -22,7 +22,7 @@
 #include <algorithm>
 #include <optional>
 
-#include <fmt/core.h>
+#include <fmt/format.h>
 
 #include <openvpn/common/platform.hpp>
 #include <openvpn/common/size.hpp>