-Wtypedef-redefinition warnings with Clang

Repeated many times, making the build very noisy. Affects Clang-based distros like FreeBSD, OpenBSD, Open Mandriva, Chimera Linux.

$ export CC=clang CXX=clang++
$ meson setup /tmp/libei_build
$ meson compile -C /tmp/libei_build
[...]
In file included from src/libei-button.c:37:
In file included from src/libei-private.h:51:
In file included from src/libei-seat.h:31:
src/ei-proto.h:22:18: warning: redefinition of typedef 'object_id_t' is a C11 feature [-Wtypedef-redefinition]
typedef uint64_t object_id_t;
                 ^
src/brei-shared.h:37:18: note: previous definition is here
typedef uint64_t object_id_t;
                 ^

Note, BSDs may use GCC 4.2 on some architectures where the warning is an error that cannot be disabled:

src/eis-proto.h:22: error: redefinition of typedef 'object_id_t'
src/brei-shared.h:37: error: previous declaration of 'object_id_t' was here