x86/tsc: Make recalibrate_cpu_khz() export GPL only

A quick search doesn't reveal any use outside of the kernel - which
would be questionable to begin with anyway - so make the export GPL
only.

Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Link: https://lore.kernel.org/r/Y599miBzWRAuOwhg@zn.tnic