Optimize positions vector allocation with reserve()

Co-authored-by: jll63 <5083077+jll63@users.noreply.github.com>

_codeql_detected_source_root

@@ -0,0 +1 @@
+.

include/boost/openmethod/policies/minimal_perfect_hash.hpp

@@ -313,6 +313,7 @@ void minimal_perfect_hash::fn<Registry>::initialize(
 
                 // Check if this displacement works for all keys in group
                 std::vector<std::size_t> positions;
+                positions.reserve(groups[g].size());
                 bool valid = true;
                 for (auto key : groups[g]) {
                     auto pilot = (mult * reinterpret_cast<detail::uintptr>(key)) >> shift;