Fix build with gcc 4.4

Report by http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=505332

/usr/bin/g++ -Wall -I. -I/usr/X11R6/include -I/usr/include/freetype2 -I/usr/include/freetype2/config -I/usr/include/libpng12 -I/usr/include -DPACKAGE=\"slim\" -DVERSION=\"1.3.0\" -DPKGDATADIR=\"/usr/share/slim\" -DSYSCONFDIR=\"/etc\" -DHAVE_SHADOW -DUSE_PAM -c switchuser.cpp -o switchuser.o
switchuser.cpp: In member function 'void SwitchUser::SetClientAuth(const char*)':
switchuser.cpp:59: error: 'remove' was not declared in this scope
make[1]: *** [switchuser.o] Error 1
make[1]: Leaving directory `/build/tbm/slim-1.3.0'
make: *** [build-stamp] Error 2

switchuser.h

@@ -18,6 +18,7 @@
 #include <pwd.h>
 #include <grp.h>
 #include <paths.h>
+#include <cstdio>
 #include <iostream>
 #include "const.h"
 #include "cfg.h"