waffle: move WAFFLE_API out of the API header

WAFFLE_API is used to define the symbol visibility and
all functions provided by the API must be non-hidden.
Thus the define should be used internally in conjunction
with the compiler used in order to properly annotate the
symbols.

Resolves #12: https://github.com/waffle-gl/waffle/issues/12
Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>
Reviewed-by: Chad Versace <chad.versace@linux.intel.com>