Fixing gr-fosphor compilation

After pulling gr-fosphor `master` today and compiling against the latest GNURadio, I noticed that it failed when attempting to link against libgnuradio, due to missing pmt symbols. The attached one-line patch fixes the issue, but I imagine would break compilation against older GNURadio releases. If there is a better way for me to submit this patch, please let me know. I had previously emailed one of the gr-fosphor developers with a separate issue regarding libpng issues out of ignorance of this mailing list. I don't want to disturb you developers any more than need be, so if there is a better forum than this for patches/pull requests, I am more than happy to use it. -E