examples: #include <cstring> in calc++

strerror is defined by <string.h>, and recent versions of GNU libstdc++
no longer include this automatically from <string>.

THANKS

@@ -3,6 +3,7 @@ it is today without the invaluable help of these people:
 
 Aaro Koskinen             aaro.koskinen@iki.fi
 Аскар Сафин               safinaskar@mail.ru
+Adam Sampson              ats@offog.org
 Airy Andre                Airy.Andre@edf.fr
 Akim Demaille             akim@lrde.epita.fr
 Albert Chin-A-Young       china@thewrittenword.com

doc/bison.texi

@@ -11810,6 +11810,7 @@ then the parser's to get the set of defined tokens.
 # include <cerrno>
 # include <climits>
 # include <cstdlib>
+# include <cstring> // strerror
 # include <string>
 # include "driver.hh"
 # include "parser.hh"