vapi: Fix build when directory contains @

vapi: Fix build when directory contains @

During build some header guards are created based on full path where
build is happening.  If one directory contains @ character build breaks
because compiler believes it's a macro declaration.

Jenkins adds `@${EXECUTOR_NUMBER}` suffix to workspace directory when
it uses more than one executor for that job, breaking the build.

Replace any @ character on guard name by _ to get it fixed.

Type: fix
Change-Id: Id0f4cfc33fda95e168541aa4e353a0d08aa3b664
Signed-off-by: Renato Botelho do Couto <renato@netgate.com>