fix non-fast string operator+ debug build

the asserts I've added need string.h , which was not available in that case

Change-Id: If8dbedcb48400e5edd0bde0214640c2c392fde2b

sal/inc/rtl/strbuf.hxx

@@ -23,6 +23,7 @@
 #include "sal/config.h"
 
 #include <cassert>
+#include <string.h>
 
 #include <rtl/strbuf.h>
 #include <rtl/string.hxx>

sal/inc/rtl/string.hxx

@@ -24,6 +24,7 @@
 
 #include <cassert>
 #include <ostream>
+#include <string.h>
 
 #include <osl/diagnose.h>
 #include <rtl/textenc.h>

sal/inc/rtl/ustrbuf.hxx

@@ -23,6 +23,7 @@
 #include "sal/config.h"
 
 #include <cassert>
+#include <string.h>
 
 #include <osl/diagnose.h>
 #include <rtl/ustrbuf.h>

sal/inc/rtl/ustring.hxx

@@ -24,6 +24,7 @@
 
 #include <cassert>
 #include <ostream>
+#include <string.h>
 
 #include "osl/diagnose.h"
 #include <rtl/ustring.h>