Fix old-style C function declarations

Fix old-style C function declarations

ges-uri-asset.c: In function ‘create_discoverer’:
ges-uri-asset.c:53:1: error: old-style function definition [-Werror=old-style-definition]
   53 | create_discoverer ()
      | ^~~~~~~~~~~~~~~~~
ges-uri-asset.c: In function ‘get_discoverer’:
ges-uri-asset.c:67:1: error: old-style function definition [-Werror=old-style-definition]
   67 | get_discoverer ()
      | ^~~~~~~~~~~~~~
  CC       libges_1.0_la-ges-auto-transition.lo
ges-asset.c: In function ‘_get_type_entries’:
ges-asset.c:489:1: error: old-style function definition [-Werror=old-style-definition]
  489 | _get_type_entries ()
      | ^~~~~~~~~~~~~~~~~