uapi/linux/const.h: Prefer ISO-friendly __typeof__

typeof is (still) a GNU extension, which means that it cannot be
used when building ISO C (e.g. -std=c99). It should therefore be
avoided in uapi headers in favour of the ISO-friendly __typeof__.

Reported-by: Ruben Ayrapetyan <ruben.ayrapetyan@arm.com>
Tested-by: Ruben Ayrapetyan <ruben.ayrapetyan@arm.com>
Signed-off-by: Kevin Brodsky <kevin.brodsky@arm.com>