powerpc/64: Include cache.h directly in paca.h

paca.h uses ____cacheline_aligned without directly including cache.h,
where it's defined.

For Book3S builds that's OK because paca.h includes lppaca.h, and it
does include cache.h.

But Book3E builds have been getting cache.h indirectly via printk.h,
which is dicey, and in fact that include was recently removed, leading
to build errors such as:

  ld: fs/isofs/dir.o:(.bss+0x0): multiple definition of `____cacheline_aligned'; fs/isofs/namei.o:(.bss+0x0): first defined here

So include cache.h directly to fix the build error.

Fixes: 534aa1dc ("printk: stop including cache.h from printk.h")
Reported-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>