Move strerror() prototype to string.h, where it belongs

src/include/errno.h

 
 extern int errno;
 
-extern const char * strerror ( int errno );
-
 struct errortab {
 	int errno;
 	const char *text;

src/include/string.h

 void * memchr(const void *s, int c, size_t n);
 char * strdup(const char *s);
 
+extern const char * strerror ( int errno );
+
 #endif /* ETHERBOOT_STRING */