configure: fix libosmo-sigtran dependency

the configure script requires libosmo-sigtran even when IU is not
enabled.

move the line that checks for libosmo-sigtran into the conditional
branch for IU

Change-Id: I72819462db0b4656fabaa80731342d06ae91fcdf
diff --git a/configure.ac b/configure.ac
index c8e23e5..0daa5e2 100644
--- a/configure.ac
+++ b/configure.ac
@@ -46,13 +46,13 @@
 PKG_CHECK_MODULES(LIBOSMOABIS, libosmoabis >= 0.2.0)
 PKG_CHECK_MODULES(LIBOSMOGB, libosmogb >= 0.6.4)
 PKG_CHECK_MODULES(LIBOSMONETIF, libosmo-netif >= 0.0.1)
-PKG_CHECK_MODULES(LIBOSMOSIGTRAN, libosmo-sigtran) # TODO version?
 PKG_CHECK_MODULES(LIBCRYPTO, libcrypto >= 0.9.5)
 
 # Enable/disable 3G aka IuPS + IuCS support?
 AC_ARG_ENABLE([iu], [AS_HELP_STRING([--enable-iu], [Build 3G support, aka IuPS and IuCS interfaces])],
     [osmo_ac_iu="$enableval"],[osmo_ac_iu="no"])
 if test "x$osmo_ac_iu" = "xyes" ; then
+    PKG_CHECK_MODULES(LIBOSMOSIGTRAN, libosmo-sigtran) # TODO version?
     PKG_CHECK_MODULES(LIBASN1C, libasn1c) # TODO version?
     PKG_CHECK_MODULES(LIBOSMORANAP, libosmo-ranap) # TODO version?
     AC_DEFINE(BUILD_IU, 1, [Define if we want to build IuPS and IuCS interfaces support])