tests/Makefile.include trying to add linking library '-lutil' that break the build on Solaris

Building script 'tests/Makefile.include' contains following code
```
ifeq ($(CONFIG_POSIX),y)
LIBS += -lutil
endif
```

library -lutil is not available on Solaris, so the building will failed, like
```
ld: fatal: library -lutil: not found
make: *** [SOMEWHERE/src/qemu-2.12.0/rules.mak:121: qemu-nbd] Error 1
```

Commenting those code out fixed the error.