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

arch/x86/kernel/tsc.c

@@ -912,8 +912,7 @@ void recalibrate_cpu_khz(void)
 						    cpu_khz_old, cpu_khz);
 #endif
 }
-
-EXPORT_SYMBOL(recalibrate_cpu_khz);
+EXPORT_SYMBOL_GPL(recalibrate_cpu_khz);
 
 
 static unsigned long long cyc2ns_suspend;