Update makefile clean file list with RPMs

`make rpms` target is producing *.rpm files in two directories:
- result/build/00-srpm-build
- result/build/01-rpm-build

This commit add those RPM files to list of the files which will
be removed during `make clean`

Makefile.am

@@ -34,7 +34,7 @@ MAINTAINERCLEANFILES =	Makefile.in config.guess config.h.in config.sub \
 						py-compile m4/* po/Makefile.in.in po/Rules-quot \
 						test-driver
 
-CLEANFILES = *~
+CLEANFILES = *~ $(RPM_BUILD_DIR)/*.rpm $(SRPM_BUILD_DIR)/*.rpm
 
 dist_noinst_DATA      = $(PACKAGE_NAME).spec