Don't depend on .git to exist, may not be present in all worktrees

arch/arm/measuremore/Kbuild

@@ -6,7 +6,7 @@ $(obj)/trigger.o: $(obj)/kernel_memo_git_commit.h $(obj)/kernel_memo_git_status.
 
 targets += $(obj)/kernel_memo_git_commit.h $(obj)/kernel_memo_git_status.h
 
-$(obj)/kernel_memo_git_commit.h: .git/HEAD .git/index
+$(obj)/kernel_memo_git_commit.h: FORCE
 	echo "const char *kernel_memo_git_commit = \"$(shell git rev-parse HEAD)\";" > $@
 
 $(obj)/kernel_memo_git_status.h: FORCE