Include unistd.h before the first use of usleep(3)

Signed-off-by: Guilherme Maciel Ferreira <guilherme.maciel.ferreira@gmail.com>

src/samples/paho_c_pub.c

@@ -51,6 +51,7 @@
 #include <unistd.h>
 #include <sys/time.h>
 #include <stdlib.h>
+#include <unistd.h>
 #endif