include/a2i/, share/mk/: Hide the inline definitions if !defined(A2I_INTERNAL)

This is another approach which doesn't need silencing the diagnostic
from GCC.

It is also more portable, since it doesn't require the GCC attribute.

Link: <https://gcc.gnu.org/bugzilla/show_bug.cgi?id=118005>
Signed-off-by: Alejandro Colomar <alx@kernel.org>