build: fix tests/prng.h missing from dist

broken by fa713d9... "zebra: rework recursive route resolution", and I
forgot to squash the fix into that.

Signed-off-by: David Lamparter <equinox@opensourcerouting.org>
diff --git a/tests/Makefile.am b/tests/Makefile.am
index e5c7fd7..9260a90 100644
--- a/tests/Makefile.am
+++ b/tests/Makefile.am
@@ -28,6 +28,8 @@
 		testprivs teststream testchecksum tabletest testnexthopiter \
 		$(TESTS_BGPD)
 
+noinst_HEADERS = prng.h
+
 testsig_SOURCES = test-sig.c
 testbuffer_SOURCES = test-buffer.c
 testmemory_SOURCES = test-memory.c