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

#include "TestSlave.h"

void TestSlave_heartbeatError(CO_Data* d, UNS8);

UNS8 TestSlave_canSend(Message *);

void TestSlave_initialisation(CO_Data* d);
void TestSlave_preOperational(CO_Data* d);
void TestSlave_operational(CO_Data* d);
void TestSlave_stopped(CO_Data* d);

void TestSlave_post_sync(CO_Data* d);
void TestSlave_post_TPDO(CO_Data* d);
void TestSlave_storeODSubIndex(CO_Data* d, UNS16 wIndex, UNS8 bSubindex);
void TestSlave_post_emcy(CO_Data* d, UNS8 nodeID, UNS16 errCode, UNS8 errReg);