Fixed pkgconfig library output

The previous output of from running "pkg-config opentxs --libs" was:

"-llibopentxs-client -llibopentxs-core -llibopentxs-ext" 

This is wrong because the library names are " libopentxs-cient, libopentxs-core, libopentxs-ext", the proper output should be:

"-lopentxs-client -lopentxs-core -lopentxs-ext"