Missing requirements on pdfio for libcupsfilters.pc

[fundawang]

Currently, libcupsfilters.pc reads:

prefix=/usr
exec_prefix=/usr
libdir=/usr/lib64
includedir=/usr/include
                                                                       
Name: libcupsfilters
Description: Library providing everything for filtering/converting print/scan job data
Version: 2.2.1

Libs: -L${libdir} -lcupsfilters
Libs.private:  -lcups -ltiff -lpng16  -lpdfio
Cflags: -I${includedir}/cupsfilters -I${includedir} -I/usr/include/p11-kit-1 -D_REENTRANT

But /usr/include/cupsfilters/pdf.h is referencing pdfio.h:

#include <stdio.h>
#include <pdfio.h>

Which means, pdfio-devel must be installed if downstream packages (like libppd) want to include pdf.h, and they might be forced to deal with pdfio structs. I guess we should add pdfio into libcupsfilters.pc:

Requires: pdfio >= 1.6.4