Link programs with pthread via cmake

All programs are now linked directly with all test code, thus adding a
pthread abstraction into the test code means having to link the programs
with pthread (if the library is found under cmake).

Signed-off-by: Paul Elliott <paul.elliott@arm.com>
diff --git a/programs/util/CMakeLists.txt b/programs/util/CMakeLists.txt
index 7fc58cb..920f69e 100644
--- a/programs/util/CMakeLists.txt
+++ b/programs/util/CMakeLists.txt
@@ -1,3 +1,5 @@
+find_package(Threads)
+
 set(libs
     ${mbedcrypto_target}
 )
@@ -9,7 +11,7 @@
 
 foreach(exe IN LISTS executables)
     add_executable(${exe} ${exe}.c $<TARGET_OBJECTS:mbedtls_test>)
-    target_link_libraries(${exe} ${libs})
+    target_link_libraries(${exe} ${libs} ${CMAKE_THREAD_LIBS_INIT})
     target_include_directories(${exe} PRIVATE ${CMAKE_CURRENT_SOURCE_DIR}/../../tests/include)
 endforeach()