build: fix wrong constextpr preventing some compilers from building biboumi

The latest commit introduced an issue because a std::string cannot
really be constexpr. Fortunately, a cosnt char* const works just as well
here.

src/database/sqlite3_engine.cpp

@@ -135,7 +135,7 @@ DatabaseType Sqlite3Engine::get_type() {
 
 bool Sqlite3Engine::is_valid_sqlite_journal_mode(std::string &sqlite_journal_mode) {
   // https://sqlite.org/pragma.html#pragma_journal_mode
-  for (const std::string &s : Sqlite3Engine::valid_sqlite_journal_modes) {
+  for (const char* s : Sqlite3Engine::valid_sqlite_journal_modes) {
     if (sqlite_journal_mode == s)
       return true;
   }