string.h: include strings.h for compatibility.

* Partly reverting hrev47655, as the moved declarations are expected
  by many ports to be accessable via string.h. 
  Following standards is a good thing in general, but not if it causes 
  more problems than it helps ...

headers/posix/string.h

@@ -74,6 +74,10 @@ extern char		*strupr(char *string);
 
 extern const char	*strsignal(int signum);
 
+/* for compatibility, pull in functions declared in strings.h */
+#include <strings.h>
+
+
 #ifdef __cplusplus
 }
 #endif