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

/* File generated by gen_cfile.py. Should not be modified. */

#ifndef WIN32TEST_H
#define WIN32TEST_H

#include "data.h"

/* Prototypes of function provided by object dictionnary */
UNS32 win32test_valueRangeTest (UNS8 typeValue, void * value);
const indextable * win32test_scanIndexOD (UNS16 wIndex, UNS32 * errorCode, ODCallback_t **callbacks);

/* Master node data struct */
extern CO_Data win32test_Data;

#endif // WIN32TEST_H