poppler-glib.pc has ${includedir}/poppler in cflags

Programs/libraries that depends on poppler-glib often assumes the base directory of poppler-glib is under the base directory of poppler, and thus does not work for nixos setup. 
By adding (poppler-glib base)/include/poppler in include path in cflags in pkg-config, this cures missing header problems. This was tested with haskell binding for poppler library.