Fix include path on cabal-1.14

Building cairo-0.12.3...
Preprocessing library cairo-0.12.3...
dist/build/Graphics/Rendering/Cairo/Matrix.h:1:26: fatal error: cairo-gtk2hs.h: No such file or directory
compilation terminated.
gtk2hsC2hs: Error during preprocessing custom header file
diff --git a/cairo.cabal b/cairo.cabal
index af7cee9..62fe84b 100644
--- a/cairo.cabal
+++ b/cairo.cabal
@@ -69,6 +69,7 @@ Library
                           Graphics.Rendering.Cairo.Internal.Surfaces.PS
                           Graphics.Rendering.Cairo.Internal.Surfaces.SVG
         extensions:     ForeignFunctionInterface
+        Include-dirs: .
         x-c2hs-Header:  cairo-gtk2hs.h
         pkgconfig-depends: cairo >= 1.2.0
         if flag(cairo_pdf)