pkgconfig: Add '-I${includedir}' to Cflags

[michaelweghorn]

Don't just specify -I${includedir}/cupsfilters,
but also -I${includedir} in the Cflags
in the pkgconfig file.

This way, includes prefixed with the "cupsfilters/" directory path also work.

This e.g. makes the libppd build work when
libcupsfilters is installed in a non-default path
(e.g. configured with --prefix=$HOME/temp/cpdb)
and libppd is configured to use that one
(e.g. by setting env variable
PKG_CONFIG_PATH=$HOME/temp/cpdb/lib/pkgconfig/
during configure).

Without this commit, libppd's configure would
succeed in the above scenario, but the actual build using make would then fail to find the cupsfilters headers:

make  all-recursive
make[1]: Entering directory '/home/michi/development/git/libppd'
make[2]: Entering directory '/home/michi/development/git/libppd'
  CC       ppd/libppd_la-ppd-attr.lo
In file included from ppd/ppd-attr.c:16:
./ppd/ppd.h:26:12: fatal error: cupsfilters/log.h: No such file or directory
   26 | #  include <cupsfilters/log.h>
      |            ^~~~~~~~~~~~~~~~~~~
compilation terminated.

(This is with libppd [1] git master as of commit
327b61cf5ec3bc74677e3b56fde4c8c4e9acdcb7).

[1] https://github.com/OpenPrinting/libppd