configure --enable-libmnl: Add libmnl to libosmocore.pc.in Requires

Change-Id: I8ad8418ada95e79bb6079f34c6b57817c6f6ab11
diff --git a/configure.ac b/configure.ac
index 1a87ec1..ee1c6f0 100644
--- a/configure.ac
+++ b/configure.ac
@@ -233,7 +233,7 @@
 	)],
 	[mnl=$enableval], [mnl="yes"])
 AS_IF([test "x$mnl" = "xyes"], [
-	PKG_CHECK_MODULES(LIBMNL, libmnl)
+	PKG_CHECK_MODULES(LIBMNL, libmnl, [AC_SUBST(LIBMNL_PC, [libmnl])])
 	AC_DEFINE([ENABLE_LIBMNL], [1], [Enable netlink socket support via libmnl])
 ])
 AM_CONDITIONAL(ENABLE_LIBMNL, test "x$mnl" = "xyes")