ethdev: fix flow API for C++

ethdev: fix flow API for C++

This commit addresses the following compilation errors:

 In file included from build/include/rte_flow_driver.h:50:0,
                  from /tmp/check-includes.sh.1397.cc:1:
 build/include/rte_flow.h:428:2: error: expected primary-expression before
    '.' token
 [...]
 build/include/rte_flow.h:469:1: sorry, unimplemented: non-trivial
    designated initializers not supported
 [...]

 In file included from build/include/rte_flow_driver.h:50:0,
                  from /tmp/check-includes.sh.1397.cc:1:
 build/include/rte_flow.h:631:1: error: C99 designator 'label_tc_s' outside
    aggregate initializer
 [...]
 build/include/rte_flow.h:631:1: error: initializer-string for array of
    chars is too long [-fpermissive]
 [...]
 build/include/rte_flow.h:650:1: sorry, unimplemented: non-trivial
    designated initializers not supported
 [...]

C++ does not support the C99-style designated initializers used in this
file for the default item masks. While the resulting symbols are primarily
useful to PMDs (written in C), they are exposed as part of the public API
for documentation purposes and to assist application writers.

Considering that:

- using pre-C99 initialization style for compatibility with C++ would
  render them difficult to understand (all struct members must be
  initialized)
- using both initialization styles would be needlessly verbose
- not exposing them at all would defeat their purpose
- applications do not normally need these symbols at run time

This commit hides these symbols from C++ applications. Specific C++
initializers will be added later if necessary.

Fixes: 6de5c0f1302c ("ethdev: define default item masks in flow API")
Fixes: 7cd048321d1d ("ethdev: add MPLS and GRE flow API items")
Cc: stable@dpdk.org
Signed-off-by: Adrien Mazarguil <adrien.mazarguil@6wind.com>
Acked-by: Beilei Xing <beilei.xing@intel.com>
Acked-by: Shahaf Shuler <shahafs@mellanox.com>