build: avoid multi-level recursion for src/ directory.

Signed-off-by: Diego Elio Pettenò <flameeyes@flameeyes.eu>
diff --git a/Makefile.am b/Makefile.am
index 794d1a2..d135e7f 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -1,7 +1,7 @@
 ACLOCAL_AMFLAGS = -I m4
 
 INCLUDES = $(all_includes) -I$(top_srcdir)/include
-SUBDIRS = include src tests utils
+SUBDIRS = include src src/vty src/codec src/gsm src/gb tests utils
 
 pkgconfigdir = $(libdir)/pkgconfig
 pkgconfig_DATA = libosmocore.pc libosmocodec.pc libosmovty.pc libosmogsm.pc \