lib/nfs/FileReader: move sys/stat.h to header because "struct stat" may be macro

It indeed is a macro on Windows.

src/lib/nfs/FileReader.cxx

@@ -31,7 +31,6 @@
 #include <assert.h>
 #include <string.h>
 #include <fcntl.h>
-#include <sys/stat.h>
 
 NfsFileReader::NfsFileReader()
 	:DeferredMonitor(io_thread_get()), state(State::INITIAL)

src/lib/nfs/FileReader.hxx

@@ -31,6 +31,7 @@
 
 #include <stdint.h>
 #include <stddef.h>
+#include <sys/stat.h>
 
 struct nfsfh;
 class NfsConnection;