libnghttp2.pc: use full name, fix Libs.private

Since the name of the file is 'libnghttp2*' I think we should use the
full name within it as well (for the Name: field), as it gets confusing
otherwise for users and for pkg-config.

Also, 'Requires.private' breaks pkg-config's ability to extract -I etc
from the Cflags below it. I believe it should be 'Libs.private' and the
right side should then list the libs with -l.

With these changes, I've successfully written a configure script to
find and use libnghttp2.