nice.pc: Include ${includedir} directly

It doesn't make sense to tell applications to include <agent.h>,
including <nice/agent.h> is much safer. So tell the applications
to use the includedir directly.

nice/nice.pc.in

@@ -10,4 +10,4 @@ Version: @VERSION@
 Requires: @NICE_PACKAGES_PUBLIC@ @GUPNP_PACKAGES_PUBLIC@
 Requires.private: @NICE_PACKAGES_PRIVATE@ @GUPNP_PACKAGES_PRIVATE@
 Libs: -L${libdir} -lnice
-Cflags: -I${includedir}/nice
+Cflags: -I${includedir}/nice -I${includedir}