eal: add macro to warn for unused function return values

eal: add macro to warn for unused function return values

This patch adds a wrapper macro __rte_warn_unused_result for the
warn_unused_result function attribute.

Marking a function __rte_warn_unused_result will make the compiler
emit a warning in case the caller does not use the function's return
value.

Signed-off-by: Mattias Rönnblom <mattias.ronnblom@ericsson.com>
Acked-by: Bruce Richardson <bruce.richardson@intel.com>
Reviewed-by: Morten Brørup <mb@smartsharesystems.com>
Acked-by: Tyler Retzlaff <roretzla@linux.microsoft.com>