Add missing include (that apparently didn't break anything on most platforms).
diff --git a/src/google/protobuf/compiler/subprocess.cc b/src/google/protobuf/compiler/subprocess.cc
index 913a1a4..dbd813e 100644
--- a/src/google/protobuf/compiler/subprocess.cc
+++ b/src/google/protobuf/compiler/subprocess.cc
@@ -34,6 +34,7 @@
 
 #ifndef _WIN32
 #include <errno.h>
+#include <sys/select.h>
 #include <sys/wait.h>
 #include <signal.h>
 #endif