Include sys/select.h for fd_set

socket.cpp fails to compile on musl with a missing definition of
fd_set.  Include sys/select.h for the definition.

Bug: 190084016
Test: m USE_HOST_MUSL=true fastboot
Change-Id: I095a56381a85981f42444f4c39ae8e7786d8640f
diff --git a/fastboot/socket.cpp b/fastboot/socket.cpp
index 5a14b63..3096905 100644
--- a/fastboot/socket.cpp
+++ b/fastboot/socket.cpp
@@ -28,6 +28,10 @@
 
 #include "socket.h"
 
+#ifndef _WIN32
+#include <sys/select.h>
+#endif
+
 #include <android-base/errors.h>
 #include <android-base/stringprintf.h>