gnutls.pc: unnecessary pathname in Libs.private

On my system (Fedora 28, gnutls-3.6.4-1.fc28.x86_64) the installed gnutls.pc file has the following line:

Libs.private: -ltspi -lgmp /usr/lib64/libunistring.so -lidn2

This shouldn't refer to the full path "/usr/lib64/libunistring.so" - it should be just "-lunistring". So if you're lucky enough to have a static version of all these libraries, static linking (gcc -static) will actually work.