ENGINES: Fix GCC warnings

[orgads]

In file included from C:/Projects/scummvm/common/array.h:25,
                 from C:/Projects/scummvm/common/achievements.h:25,
                 from C:/Projects/scummvm/engines/metaengine.h:25,
                 from C:/Projects/scummvm/engines/advancedDetector.h:25,
                 from C:/Projects/scummvm/engines/sci/detection.cpp:22:
C:/Projects/scummvm/common/scummsys.h:400:44: warning: attribute ignored [-Wattributes]
  400 |                 #define WARN_UNUSED_RESULT [[nodiscard]]
      |                                            ^
C:/Projects/scummvm/engines/metaengine.h:565:16: note: in expansion of macro 'WARN_UNUSED_RESULT'
  565 |         static WARN_UNUSED_RESULT bool readSavegameHeader(Common::InSaveFile *in, ExtendedSavegameHeader *header, bool skipThumbnail = true);
      |                ^~~~~~~~~~~~~~~~~~
C:/Projects/scummvm/common/scummsys.h:400:44: note: an attribute that appertains to a type-specifier is ignored
  400 |                 #define WARN_UNUSED_RESULT [[nodiscard]]
      |                                            ^
C:/Projects/scummvm/engines/metaengine.h:565:16: note: in expansion of macro 'WARN_UNUSED_RESULT'
  565 |         static WARN_UNUSED_RESULT bool readSavegameHeader(Common::InSaveFile *in, ExtendedSavegameHeader *header, bool skipThumbnail = true);
      |                ^~~~~~~~~~~~~~~~~~

[bluegr]

Nice work!

Here are some relevant resources on this:

• https://stackoverflow.com/questions/29807010/using-gcc-attributes-with-c11-attributes-syntax
• https://en.cppreference.com/w/cpp/language/attributes

Merging