Makefile.am: Do not remove ChangeLog on make distclean, fixes #116

[pbatard: Remove it on make maintainer-clean instead]
diff --git a/Makefile.am b/Makefile.am
index 1b65a92..3db0b72 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -1,6 +1,7 @@
 AUTOMAKE_OPTIONS = dist-bzip2 no-dist-gzip
 ACLOCAL_AMFLAGS = -I m4
-DISTCLEANFILES = ChangeLog libusb-1.0.pc
+DISTCLEANFILES = libusb-1.0.pc
+MAINTAINERCLEANFILES = ChangeLog
 EXTRA_DIST = TODO PORTING
 SUBDIRS = libusb doc