xz: Include <strings.h> in suffix.c if needed for strcasecmp().

SUSv2 and POSIX.1‐2017 declare only a few functions in <strings.h>.
Of these, strcasecmp() is used on some platforms in suffix.c.
Nothing else in the project needs <strings.h> (at least if
building on a modern system).

sysdefs.h currently includes <strings.h> if HAVE_STRINGS_H is
defined and suffix.c relied on this.

Note that dos/config.h doesn't #define HAVE_STRINGS_H even though
DJGPP does have strings.h. It isn't needed with DJGPP as strcasecmp()
is also in <string.h> in DJGPP.

src/xz/suffix.c

@@ -18,6 +18,9 @@
 
 // For case-insensitive filename suffix on case-insensitive systems
 #if defined(TUKLIB_DOSLIKE) || defined(__VMS)
+#	ifdef HAVE_STRINGS_H
+#		include <strings.h>
+#	endif
 #	define strcmp strcasecmp
 #endif