(maint) add libdl explicitly

Prior to this commit, building cfacter would result in the following
error:
```
../libfacter.so.0.2.0: undefined reference to `dlopen'
../libfacter.so.0.2.0: undefined reference to `dlclose'
../libfacter.so.0.2.0: undefined reference to `dlsym'
collect2: error: ld returned 1 exit status
```
which indicates we are not linking that library correctly, if at
all.

I was told that this variable should be changed to "PLATFORM_LIBS", but
I can't get that to work, whereas this does. If there's a better way to
fix this error, I'd definitely be open to figuring that out.