Install programs into $(prefix)/bin, not /sbin

At least in many configurations, there is no need to run any
of our programs as root.  Thus, we shouldn't install them in sbin.
diff --git a/openbsc/src/gprs/Makefile.am b/openbsc/src/gprs/Makefile.am
index fe34494..2fa9181 100644
--- a/openbsc/src/gprs/Makefile.am
+++ b/openbsc/src/gprs/Makefile.am
@@ -6,9 +6,9 @@
 noinst_HEADERS = gprs_sndcp.h
 
 if HAVE_LIBGTP
-sbin_PROGRAMS = osmo-gbproxy osmo-sgsn
+bin_PROGRAMS = osmo-gbproxy osmo-sgsn
 else
-sbin_PROGRAMS = osmo-gbproxy
+bin_PROGRAMS = osmo-gbproxy
 endif