tests/Makefile.am: clean up AM_* variables

* -I is a preprocessor flag, so it should be in AM_CPPFLAGS
* $(LIBOSMOCORE_LDFLAGS) is not defined here, remove it

Change-Id: Ice892f848e06b601e67b06a4e1e25c11fb7c4128

tests/Makefile.am

@@ -1,5 +1,6 @@
-AM_CFLAGS = -Wall -I$(top_srcdir)/include $(LIBOSMOCORE_CFLAGS) -g
-AM_LDFLAGS = $(LIBOSMOCORE_LDFLAGS) -no-install
+AM_CPPFLAGS = -I$(top_srcdir)/include
+AM_CFLAGS = -Wall $(LIBOSMOCORE_CFLAGS) -g
+AM_LDFLAGS = -no-install
 
 LDADD = \
 	$(top_builddir)/src/libosmonetif.la \