Standardize poppler header names

Ran into a strange issue trying to compile Inkscape with a local poppler build (the recent spring (winter?) cleaning removed the CFlags I was using earlier) --- and I had to fiddle with the local poppler.pc for it to work. I think this is because of the way headers are called:

Eg. inkscape-version-info.cpp uses #include <poppler-config.h> while pdf-input.cpp uses #include <poppler/GfxFont.h> despite both the header files (poppler-config.h and GfxFont.h) being in the same directory.

The usage is inconsistent (sometimes within the same file); but the latter, it seems, isn't 'recognized' by poppler's bundled pkg-config file. I've removed every poppler/ from the header files, and everything works (afaict), even without pkg-config shenanigans. I'm not sure if this is right, though. Maybe it's the upstream pkg-config that needs changing?