vpp: binary-api CLI weak linking workaround

For some unknown reason sometimes calling exec() ends up on weak
exec() defined in src/vat/api_format.c which return -1 instead of using
one few lines above.

Another proof that use of weak symbols is bad idea.

Luckily this can be easily workarounded.

Type: fix
Change-Id: Ic84e8525bff75c1b8186c233cd524aac4d95c8b5
Signed-off-by: Damjan Marion <damarion@cisco.com>