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;
   }

src/database/sqlite3_engine.hpp

@@ -48,7 +48,7 @@ private:
       // https://sqlite.org/lang_analyze.html
       "PRAGMA optimize=0x10002",
   };
-  constexpr static std::string valid_sqlite_journal_modes[6] = {
+  constexpr static const char *const valid_sqlite_journal_modes[6] = {
       "DELETE", "TRUNCATE", "PERSIST", "MEMORY", "WAL", "OFF",
   };
   static bool is_valid_sqlite_journal_mode(std::string &sqlite_journal_mode);