test suite LDLIBS: link with libpthread on GNU/Linux

I needed this after some package updating on my distribution.  It is harmless
anyway, even where not needed.

test/Makefile

@@ -48,7 +48,7 @@ endif
 LDLIBS+= -lasn1
 
 ifeq ($(BUILD_FOR),Linux)
-    LDLIBS+= -luuid
+    LDLIBS+= -luuid -lpthread
 endif
 
 ifeq ($(OPENPGP),SEQUOIA)