eventdev: fix headers for strict compilation flags

eventdev: fix headers for strict compilation flags

Exported headers must allow compilation with the strictest flags. This
commit addresses the following errors:

 In file included from build/include/rte_eventdev_pmd.h:55:0,
                  from /tmp/check-includes.sh.25816.c:1:
 build/include/rte_eventdev.h:908:8: error: struct has no named members
    [-Werror=pedantic]
 [...]
 In file included from /tmp/check-includes.sh.25816.c:1:0:
 build/include/rte_eventdev_pmd.h:65:35: error: ISO C does not permit named
    variadic macros [-Werror=variadic-macros]
 [...]

Fixes: 71f238432865 ("eventdev: introduce event driven programming model")
Fixes: 4f0804bbdfb9 ("eventdev: implement the northbound APIs")

Signed-off-by: Adrien Mazarguil <adrien.mazarguil@6wind.com>
Acked-by: Jerin Jacob <jerin.jacob@caviumnetworks.com>