Makefile.am: Fix typo

Change-Id: I392cf453087230d848bafe959544646e68df8070
diff --git a/Makefile.am b/Makefile.am
index 354e972..9ad5947 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -13,7 +13,7 @@
 
 AM_LDFLAGS = \
 	$(COVERAGE_LDFLAGS) \
-	$(NUKK)
+	$(NULL)
 
 bin_PROGRAMS = \
 	osmo-sysmon \