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

Source: canfestival
Section: devel
Priority: standard
Maintainer: edouard <edouard.tisserant@gmail.com>
Build-Depends: debhelper (>= 5), peak-linux-driver-modules
Standards-Version: 3.7.2

Package: canfestival-peak
Architecture: all
Depends: canfestival-devel, canfestival-objdictedit
Description: Can Driver Interface for Peak Driver