x86/vdso: Add missing brackets in switch case

0-day reported:

  arch/x86/entry/vdso/vma.c:199:3: warning: label followed by a declaration
  				 	    is a C23 extension [-Wc23-extensions]

Add the missing brackets.

Fixes: e93d2521 ("x86/vdso: Split virtual clock pages into dedicated mapping")
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Closes: https://lore.kernel.org/oe-kbuild-all/202411022359.fBPFTg2T-lkp@intel.com/