x86/sev: Include fpu/xcr.h

Include the header which only provides the XCR accessors. That's all what
is needed here.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lkml.kernel.org/r/20211015011539.896573039@linutronix.de