build: use nobase_include_HEADERS instead of using recursive Makefiles

This simplifies installation of include files, as make no longer have
to recurse into subdirectories and install each subset of files in
different directories.

Signed-off-by: Diego Elio Pettenò <flameeyes@flameeyes.eu>
diff --git a/include/Makefile.am b/include/Makefile.am
index ea38eec..61e6cf5 100644
--- a/include/Makefile.am
+++ b/include/Makefile.am
@@ -1,3 +1,6 @@
 noinst_HEADERS=mISDNif.h internal.h
 
-SUBDIRS=osmocom
+nobase_include_HEADERS = osmocom/abis/ipa.h osmocom/abis/trau_frame.h	\
+ osmocom/abis/ipa_proxy.h osmocom/abis/ipaccess.h osmocom/abis/abis.h	\
+ osmocom/abis/subchan_demux.h osmocom/abis/e1_input.h			\
+ osmocom/abis/lapd.h osmocom/trau/osmo_ortp.h