build: Make sure plist.pxd is included in tarballs

When generating a tarball on a system without cython installed,
plist.pxd was missing, causing build failures later on when trying to
use cython.
This seems to be caused by EXTRA_DIST being inside a if HAVE_CYTHON
block rather than being outside of it.
diff --git a/cython/Makefile.am b/cython/Makefile.am
index 544428d..fe4330e 100644
--- a/cython/Makefile.am
+++ b/cython/Makefile.am
@@ -3,19 +3,18 @@
 AM_CFLAGS = $(GLOBAL_CFLAGS)
 AM_LDFLAGS = $(GLOBAL_LDFLAGS)
 
+EXTRA_DIST = plist.pyx plist.pxd
+
 if HAVE_CYTHON
 
 BUILT_SOURCES = plist.c
 PXDINCLUDES = plist.pxd $(CYTHON_PLIST_INCLUDE_DIR)/plist.pxd
-PXIINCLUDES =
 
 CLEANFILES =                \
         *.pyc               \
         *.pyo               \
         plist.c
 
-EXTRA_DIST = plist.pyx plist.pxd $(PXIINCLUDES)
-
 plistdir = $(pyexecdir)
 plist_LTLIBRARIES = plist.la
 plist_la_SOURCES = plist_util.c plist_util.h plist.pyx