src/Makefile.am: Build src directory (libosmo-rspro) before sub-directories

This ensures that libosmo-rspro is first built/installed, before the
programs that depend on it are.

This resolves warnings like
libtool: warning: '../../src/libosmo-rspro.la' has not been installed in '/tmp/osmo-remsim/osmo-remsim-0.2.2.59-2f5c/_inst/lib'
during the build process.

Change-Id: I928ff36406986cd0d3fe493f258864a38ce37798
diff --git a/src/Makefile.am b/src/Makefile.am
index c954db2..6fff228 100644
--- a/src/Makefile.am
+++ b/src/Makefile.am
@@ -1,4 +1,4 @@
-SUBDIRS = rspro
+SUBDIRS = rspro .
 if BUILD_BANKD
 SUBDIRS += bankd
 endif