Adjust linux_vdso_{cpu,tsc}_selector_idx() definitions to avoid clang 15 warnings

Adjust linux_vdso_{cpu,tsc}_selector_idx() definitions to avoid clang 15 warnings

With clang 15, the following -Werror warnings are produced:

    sys/x86/linux/linux_vdso_selector_x86.c:44:28: error: a function declaration without a prototype is deprecated in all versions of C [-Werror,-Wstrict-prototypes]
    linux_vdso_tsc_selector_idx()
                               ^
                                void
    sys/x86/linux/linux_vdso_selector_x86.c:62:28: error: a function declaration without a prototype is deprecated in all versions of C [-Werror,-Wstrict-prototypes]
    linux_vdso_cpu_selector_idx()
                               ^
                                void

This is because linux_vdso_tsc_selector_idx() and
linux_vdso_cpu_selector_idx are declared with (void) argument lists, but
defined with empty argument lists. Make the definitions match the
declarations.

MFC after:	3 days