Adjust agp_find_device() definition in agp.c to avoid clang 15 warning

With clang 15, the following -Werror warning is produced:

sys/dev/agp/agp.c:910:16: error: a function declaration without a prototype is deprecated in all versions of C [-Werror,-Wstrict-prototypes]
agp_find_device()
               ^
                void

This is because agp_find_device() is declared with a (void) argument
list, and defined with an empty argument list. Make the definition match
the declaration.

MFC after: 3 days

(cherry picked from commit c0817e2aba26ceb683e9c649c22d7fcd9a921231)