linux-atm: add patch to simplify build

package/linux-atm/patches/patch-src_Makefile_am

@@ -0,0 +1,7 @@
+--- linux-atm-2.5.1.orig/src/Makefile.am	2009-08-03 22:36:56.000000000 +0200
++++ linux-atm-2.5.1/src/Makefile.am	2020-09-14 15:09:51.650818873 +0200
+@@ -1,3 +1 @@
+-SUBDIRS = include lib test debug qgen q2931 saal sigd maint arpd ilmid man led lane \
+-		mpoad oamd switch config extra br2684
+-
++SUBDIRS = include lib br2684ctl