.git-blame-ignore-revs: Fix hash for sys/kern/kern_cpu.c whitespace changes

Really fill in the hash of the MFCed commit, removing the placeholder
I forgot to update before commit.

This is a direct commit to stable/15.

Fixes:          bd13516d40 (".git-blame-ignore-revs: sys/kern/kern_cpu.c whitespace changes")
Sponsored by:   The FreeBSD Foundation

.git-blame-ignore-revs

@@ -9,7 +9,7 @@
 # This file is sorted in git log order (newest commits first).
 
 # sys/kern/kern_cpu.c whitespace
-XXX
+22f80045a1869c3dd66aef0284fae32093ba5816
 # witness white space cleanup and style(9) tweeks
 f5377665253b2b107ee8a4690ad2e6682375b304
 # style fixes for device_if.m