netfilter: ebtables: use __u8 instead of uint8_t in uapi header

When CONFIG_UAPI_HEADER_TEST=y, exported headers are compile-tested to
make sure they can be included from user-space.

Currently, linux/netfilter_bridge/ebtables.h is excluded from the test
coverage. To make it join the compile-test, we need to fix the build
errors attached below.

For a case like this, we decided to use __u{8,16,32,64} variable types
in this discussion:

  https://lkml.org/lkml/2019/6/5/18



Build log:

  CC      usr/include/linux/netfilter_bridge/ebtables.h.s
In file included from <command-line>:32:0:
./usr/include/linux/netfilter_bridge/ebtables.h:126:4: error: unknown type name ‘uint8_t’
    uint8_t revision;
    ^~~~~~~
./usr/include/linux/netfilter_bridge/ebtables.h:139:4: error: unknown type name ‘uint8_t’
    uint8_t revision;
    ^~~~~~~
./usr/include/linux/netfilter_bridge/ebtables.h:152:4: error: unknown type name ‘uint8_t’
    uint8_t revision;
    ^~~~~~~

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>