gpgme.pc: Fix library dependency and use of includedir, libdir.

* src/gpgme.pc.in (Require): Move gpg-error, libassuan to...
(Requires.private): ... here.
(Cflags): Add -I${includedir}.
(Libs): Add -L${libdir}.

This will result better library dependency for applications using
gpgme. It will break build of an application which directly uses
gpg-error or libassuan without specifying them (which means, depending
on redundant/wrong gpgme.pc).

• Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>