strings.h: add #include required when strict

Due to use of strcasecmp() or similar C API.

Maybe HAVE_STRINGS_H is needed ?  If so which platforms needs this ?

clang18 default warning cleanup (strict)

ext2sim/ext2sim.c

@@ -19,6 +19,7 @@ static char rcsid[] __attribute__ ((unused)) = "$Header: /usr/cvsroot/magic-8.0/
 #include <stdio.h>
 #include <stdlib.h>	/* for atof() */
 #include <string.h>
+#include <strings.h>
 #include <ctype.h>
 #include <math.h>	/* for sqrt() in bipolar L,W calculation */