bugfix: disabled dtrace in the libuv build because it caused issues when recent versions of systemtap is installed

Without this patch, the build fails on my side with recent versions of systemtap installed in the same system:

```
make[1]: Entering directory `/home/agentzh/git/cf/nginx-fl/lua/keyless/tmp/libuv-1daff47ae9df55902f07d3c5b8a3a393306a2f1e'
GEN include/uv-dtrace.h
/opt/systemtap/bin/dtrace invalid option -xnolibs
Usage /opt/systemtap/bin/dtrace [--help] [-h | -G] [-C [-I<Path>]] -s File.d [-o <File>]
make[1]: *** [include/uv-dtrace.h] Error 1
make[1]: Leaving directory `/home/agentzh/git/cf/nginx-fl/lua/keyless/tmp/libuv-1daff47ae9df55902f07d3c5b8a3a393306a2f1e'
make: *** [tmp/libuv-1daff47ae9df55902f07d3c5b8a3a393306a2f1e/.libs/libuv.a] Error 2
```

Basically, the `dtrace` utility from recent versions of systemtap no longer supports the `-xnolibs` option.