Use API version in cython libplist include directory check to fix linking

This was the last bit that was still linking to former non API version
libplist if it was also installed on the system.
diff --git a/configure.ac b/configure.ac
index b928e0a..78f96ec 100644
--- a/configure.ac
+++ b/configure.ac
@@ -135,7 +135,7 @@
 if [test "x$CYTHON" != "xfalse"]; then
             PKG_PROG_PKG_CONFIG
             AC_MSG_CHECKING([for libplist Cython bindings])
-            CYTHON_PLIST_INCLUDE_DIR=$($PKG_CONFIG --variable=includedir libplist)/plist/cython
+            CYTHON_PLIST_INCLUDE_DIR=$($PKG_CONFIG --variable=includedir libplist-2.0)/plist/cython
             if [test ! -d "$CYTHON_PLIST_INCLUDE_DIR"]; then
                 CYTHON_PLIST_INCLUDE_DIR=.
                 cython_python_bindings=yes