Fix odr_check visibility: add BOOST_SYMBOL_VISIBLE to prevent false ODR violation with dynamic loading

When a project uses <visibility>hidden (which adds -fvisibility-inlines-hidden),
template static variable initializers like odr_check::inc<R> get hidden symbol
visibility. When multiple shared libraries are loaded via RTLD_GLOBAL on ELF
systems (e.g. s390x), hidden symbols are not deduplicated by the dynamic linker.
Each DSO initializes its own copy of inc<R>, each time incrementing the shared
count variable. When count > 1, initialize() falsely fires the ODR violation check.

Fix: add BOOST_SYMBOL_VISIBLE to odr_check struct, which expands to
__attribute__((__visibility__("default"))) on GCC/Clang (no-op on MSVC/Windows).
This overrides -fvisibility-inlines-hidden for all struct members including the
inc<R> template static, ensuring proper RTLD_GLOBAL symbol deduplication.

Also add the required #include <boost/config.hpp> for BOOST_SYMBOL_VISIBLE.

Author: Copilot

include/boost/openmethod/preamble.hpp

@@ -108,7 +108,7 @@ struct odr_violation : openmethod_error {
 namespace detail {
 
 template<class Registry>
-struct odr_check {
+struct BOOST_SYMBOL_VISIBLE odr_check {
     static std::size_t count;
     template<class R>
     static std::size_t inc;