Add proper include for definitions of EXIT_* constants

These constants were referenced in src/client/main.cpp but their
definitions were quite properly pulled indirectly through other
header files. On some toolchains they were not, which broke
compilation.

Signed-off-by: Alexander Lukichev <alexander.lukichev@gmail.com>
Merge pull request #1 from alukichev/develop

Add proper include for definitions of EXIT_* constants