Fix minor warnings

tests/parallel_tasks/explicit_combined_worker.c

@@ -17,6 +17,7 @@
 
 #include <starpu.h>
 #include <limits.h>
+#include <unistd.h>
 
 #define N	1000
 #define VECTORSIZE	1024

tests/parallel_tasks/parallel_kernels.c

@@ -17,6 +17,7 @@
 
 #include <starpu.h>
 #include <limits.h>
+#include <unistd.h>
 
 #define N	1000
 #define VECTORSIZE	1024

tests/parallel_tasks/parallel_kernels_spmd.c

@@ -17,6 +17,7 @@
 
 #include <starpu.h>
 #include <limits.h>
+#include <unistd.h>
 
 #define N	1000
 #define VECTORSIZE	1024

tests/parallel_tasks/spmd_pgreedy.c

@@ -16,6 +16,7 @@
 
 #include <starpu.h>
 #include <limits.h>
+#include <unistd.h>
 
 #define N	1000
 #define VECTORSIZE	1024