Adjust vt_mouse_paste() definition to avoid clang 15 warning

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

sys/dev/vt/vt_core.c:2129:15: error: a function declaration without a prototype is deprecated in all versions of C [-Werror,-Wstrict-prototypes]
vt_mouse_paste()
              ^
               void

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

MFC after: 3 days