Include the local include directory first.

This fixes a problem when libharu in built on a system where a previous version
of libharu is installed and the structures have changed, e.g.,

[  4%] Building C object src/CMakeFiles/hpdf.dir/hpdf_annotation.o
/usr/ports/print/libharu/work/libharu-libharu-7f4bb51/src/hpdf_annotation.c:235:46:
error: use of undeclared identifier 'HPDF_ANNOT_WIDGET'; did you mean
'HPDF_ANNOT_HIDDEN'?
    annot = HPDF_Annotation_New (mmgr, xref, HPDF_ANNOT_WIDGET, rect);
                                             ^~~~~~~~~~~~~~~~~
                                             HPDF_ANNOT_HIDDEN
/usr/local/include/hpdf_types.h:366:5: note: 'HPDF_ANNOT_HIDDEN'
declared here
    HPDF_ANNOT_HIDDEN,
    ^