binder: fix sender_euid type in uapi header

The {pid,uid}_t fields of struct binder_transaction were recently
replaced to use kernel types in commit 169adc2b ("android/binder.h:
add linux/android/binder(fs).h to UAPI compile-test coverage").

However, using __kernel_uid_t here breaks backwards compatibility in
architectures using 16-bits for this type, since glibc and some others
still expect a 32-bit uid_t. Instead, let's use __kernel_uid32_t which
avoids this compatibility problem.

Fixes: 169adc2b ("android/binder.h: add linux/android/binder(fs).h to UAPI compile-test coverage")
Reported-by: Christopher Ferris <cferris@google.com>
Signed-off-by: Carlos Llamas <cmllamas@google.com>
Acked-by: Todd Kjos <tkjos@google.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>