Makefile: fix make mapistore test

Following cookbook recipe we face this error while executing make:

    vagrant@precise64:~/openchange$ make
    Linking bin/mapistore_test
    /usr/bin/ld: warning: libmapi.so.0, needed by mapiproxy/libmapistore.so.2.2, not found (try using -rpath or -rpath-link)
    mapiproxy/libmapistore.so.2.2: undefined reference to `exchange_globcnt'
    mapiproxy/libmapiproxy.so.2.2: undefined reference to `mapi_get_locale_from_lcid'
    mapiproxy/libmapiproxy.so.2.2: undefined reference to `set_errno'
    collect2: ld returned 1 exit status

http://tracker.openchange.org/boards/3/topics/306
You should be able to remove libmapi.$(SHLIBEXT).$(PACKAGE_VERSION) from the linker flags now, since it's already coming in through $^