Add missing includes for pid_t.

Was being transitively included through libc++, but the libc++ update
fixes some of the leaky headers (used to always include unistd.h!).

Change-Id: I40caf74695bb75901bda24665b8d83a6914de9c4
diff --git a/simpleperf/environment.h b/simpleperf/environment.h
index aa6f5eb..a0c085d 100644
--- a/simpleperf/environment.h
+++ b/simpleperf/environment.h
@@ -17,6 +17,8 @@
 #ifndef SIMPLE_PERF_ENVIRONMENT_H_
 #define SIMPLE_PERF_ENVIRONMENT_H_
 
+#include <sys/types.h>
+
 #include <functional>
 #include <set>
 #include <string>
diff --git a/simpleperf/record.h b/simpleperf/record.h
index c63a38e..c3791b4 100644
--- a/simpleperf/record.h
+++ b/simpleperf/record.h
@@ -17,6 +17,8 @@
 #ifndef SIMPLE_PERF_RECORD_H_
 #define SIMPLE_PERF_RECORD_H_
 
+#include <sys/types.h>
+
 #include <string>
 #include <vector>