configure/Makefile: Include contrib in "make dist"

Change-Id: I2d6a2e06f075391e8e51d1342a1afe955cc127b0
diff --git a/Makefile.am b/Makefile.am
index 9789ba3..9c51a55 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -1,6 +1,6 @@
 AUTOMAKE_OPTIONS = foreign dist-bzip2
 
-SUBDIRS = src include doc
+SUBDIRS = contrib src include doc
 
 pkgconfigdir = $(libdir)/pkgconfig
 pkgconfig_DATA = libosmo-rspro.pc