gb: suspend_ref was changed to a plain uint8_t, update the code

Require a 0.5.2+ libosmogb due the change of the suspend_ref type.
diff --git a/openbsc/configure.ac b/openbsc/configure.ac
index 3f2d4d4..8284b88 100644
--- a/openbsc/configure.ac
+++ b/openbsc/configure.ac
@@ -45,7 +45,7 @@
 PKG_CHECK_MODULES(LIBOSMOVTY, libosmovty >= 0.3.0)
 PKG_CHECK_MODULES(LIBOSMOGSM, libosmogsm >= 0.3.0)
 PKG_CHECK_MODULES(LIBOSMOABIS, libosmoabis >= 0.1.0)
-PKG_CHECK_MODULES(LIBOSMOGB, libosmogb >= 0.5.1)
+PKG_CHECK_MODULES(LIBOSMOGB, libosmogb >= 0.5.2)
 
 found_libgtp=yes
 PKG_CHECK_MODULES(LIBGTP, libgtp, , found_libgtp=no)