including gtk/gtkunixprint.h appears unnecessary

...and at least gtk2-devel-2.10.4-30.el5, while new enough to cause
ENABLE_GTK_PRINT=TRUE, does not contain
/usr/include/gtk-unix-print-2.0/gtk/gtkunixprint.h

Change-Id: I6ff98e0c67047c0d8c35cfcad45be47ff09ca97a