Install expat_config.h as well

https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=302191
diff --git a/expat/Makefile.in b/expat/Makefile.in
index 9c0f5d4..b791742 100644
--- a/expat/Makefile.in
+++ b/expat/Makefile.in
@@ -42,7 +42,7 @@
 mkinstalldirs = $(SHELL) $(top_srcdir)/conftools/mkinstalldirs
 
 MANFILE = $(srcdir)/doc/xmlwf.1
-APIHEADER = $(srcdir)/lib/expat.h $(srcdir)/lib/expat_external.h
+APIHEADER = $(srcdir)/lib/expat.h $(srcdir)/lib/expat_external.h expat_config.h
 LIBRARY = libexpat.la
 
 DESTDIR = $(INSTALL_ROOT)