Fix build with cups 1.5 snapshots

Fix build with cups 1.5 snapshots

Without this patch, Qt CUPS support fails to build with current
CUPS snapshots:

In file included from ../../include/QtGui/private/qcups_p.h:1:0,
                 from painting/qpdf.cpp:47:
.../qcups_p.h:78:11: error: 'ppd_file_t' does not name a type
.../qcups_p.h:80:11: error: 'ppd_file_t' does not name a type
.../qcups_p.h:81:11: error: 'ppd_option_t' does not name a type
.../qcups_p.h:84:11: error: 'ppd_option_t' does not name a type
.../qcups_p.h:87:34: error: ISO C++ forbids declaration of 'type
                            name' with no type
.../qcups_p.h:87:47: error: template argument 1 is invalid
.../qcups_p.h:103:56: error: 'ppd_group_t' does not name a type
.../qcups_p.h:103:77: error: ISO C++ forbids declaration of 'group'
                      with no type
.../qcups_p.h:104:62: error: 'ppd_group_t' does not name a type
.../qcups_p.h:104:75: error: ISO C++ forbids declaration of 'group'
                      with no type
.../qcups_p.h:108:11: error: 'ppd_option_t' does not name a type
.../qcups_p.h:110:5: error: 'ppd_file_t' does not name a type

Merge-request: 835
Reviewed-by: Olivier Goffart <olivier.goffart@nokia.com>