Fix : libcanfestival_unix.a not built if examples are not compiled

Problem: when building Canfestival with unix timers 'libcanfestiva_unix.a'
is built only if you build examples too.

Test case:

$ ./configure --timers=unix --can=socket --target=unix
$ make canfestival

...

no errors but 'libcanfestival_unix.a' is not built:
$ find . -name "*.a"
./src/libcanfestival.a

Changing line 90 of 'drivers/unix/Makefile.in' to:
#driver: $(OBJS)
driver: libcanfestival_$(TARGET).a

solves the problem:

$ ./configure --timers=unix --can=socket --target=unix
$ make canfestival

...

$ find . -name "*.a"
./drivers/unix/libcanfestival_unix.a
./src/libcanfestival.a

#ifndef CALLBACK_H_
#define CALLBACK_H_

extern "C"
{
#include "ObjDict.h"
}

void Call_heartbeatError (CO_Data * d, UNS8);

UNS8 Call_canSend (Message *);

void Call_initialisation (CO_Data * d);
void Call_preOperational (CO_Data * d);
void Call_operational (CO_Data * d);
void Call_stopped (CO_Data * d);

void Call_post_sync (CO_Data * d);
void Call_post_TPDO (CO_Data * d);
void Call_storeODSubIndex (CO_Data * d, UNS16 wIndex, UNS8 bSubindex);

#endif /*CALLBACK_H_ */