libsshpp: include required <string> header for std::string

Signed-off-by: Petar Koretic <petar.koretic@sartura.hr>
Reviewed-by: Andreas Schneider <asn@cryptomilk.org>

include/libssh/libsshpp.hpp

@@ -56,6 +56,7 @@
 #include <stdlib.h>
 #include <stdarg.h>
 #include <stdio.h>
+#include <string>
 
 namespace ssh {