Fix subprocess Werror

[aquette]

preferably move to fty-common

CXX src/src_libfty_discovery_la-subprocess.lo

In file included from src/subprocess.cc:29:0:

src/fty_discovery_classes.h:103:5: error: 'void subprocess_test(bool)': visibility attribute ignored because it [-Werror=attributes]

 subprocess_test (bool verbose);

 ^

In file included from src/fty_discovery_classes.h:82:0,

             from src/subprocess.cc:29:

src/subprocess.h:205:5: error: conflicts with previous declaration here [-Werror=attributes]

 subprocess_test (bool verbose);

[jimklimov]

Hopefully #52 will fix this