perf data: Add missing unistd.h header needed for pid_t

Noticed when processing 'perf kwork' that includes util/data.h without,
by luck, having included unistd.h indirectly to get the pid_t typedef.
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/util/data.h

@@ -4,6 +4,7 @@
 
 #include <stdio.h>
 #include <stdbool.h>
+#include <unistd.h>
 #include <linux/types.h>
 
 enum perf_data_mode {