tests: fix Makefile.am so it works with BSD make

The export statement is specific to GNU make and breaks the build
with BSD make. I couldn't observe any difference in behaviour
between having the export present and absent, therefore, just remove it.

Signed-off-by: Christian Franke <chris@opensourcerouting.org>
diff --git a/tests/Makefile.am b/tests/Makefile.am
index 93c06b0..b7a7fe7 100644
--- a/tests/Makefile.am
+++ b/tests/Makefile.am
@@ -1,5 +1,4 @@
 AUTOMAKE_OPTIONS    = dejagnu
-export DEJAGNU
 DEJATOOL = libzebra
 
 SUBDIRS = \