um: elf.h: Fix W=1 warning for empty body in 'do' statement

Use the common kernel style to eliminate a warning:

./arch/x86/um/asm/elf.h:215:32: warning: suggest braces around empty body in ‘do’ statement [-Wempty-body]
 #define SET_PERSONALITY(ex) do ; while(0)
                                ^

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Jeff Dike <jdike@addtoit.com>
Cc: Richard Weinberger <richard@nod.at>
Cc: Anton Ivanov <anton.ivanov@cambridgegreys.com>
Cc: linux-um@lists.infradead.org
Signed-off-by: Richard Weinberger <richard@nod.at>