Explicitly link tests with pthreads

Required to use pthreads within tests.

Signed-off-by: Paul Elliott <paul.elliott@arm.com>
diff --git a/tests/Makefile b/tests/Makefile
index 2249a55..bcc3b93 100644
--- a/tests/Makefile
+++ b/tests/Makefile
@@ -57,6 +57,7 @@
 DLEXT ?= so
 EXEXT=
 SHARED_SUFFIX=
+LOCAL_LDFLAGS += -lpthread
 endif
 
 ifdef WINDOWS