cygwin: explicitly include required c headers

src/libstore/build.cc

@@ -20,6 +20,7 @@
 #include <sys/types.h>
 #include <sys/stat.h>
 #include <sys/utsname.h>
+#include <sys/select.h>
 #include <fcntl.h>
 #include <unistd.h>
 #include <errno.h>

src/libstore/local-store.cc

@@ -13,6 +13,7 @@
 
 #include <sys/types.h>
 #include <sys/stat.h>
+#include <sys/select.h>
 #include <sys/time.h>
 #include <unistd.h>
 #include <utime.h>