Adjust playinit() definition to avoid clang 15 warning

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

sys/dev/speaker/spkr.c:182:9: error: a function declaration without a prototype is deprecated in all versions of C [-Werror,-Wstrict-prototypes]
playinit()
        ^
         void

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

MFC after: 3 days

(cherry picked from commit 57c46916e145581807333bc06a08e9532721bd8f)