m4: Move byteorder.m4 back to m4/ directory

There is little value in putting this in its own directory just
because it is a non-generated file.

.gitignore

@@ -30,6 +30,7 @@ Makefile.in
 /ltmain.sh
 /ltmain.sh.orig
 /m4/
+!/m4/byteorder.m4
 /missing
 /mkinstalldirs
 /test-driver

acinclude.m4

@@ -760,4 +760,4 @@ void vsyslog(int priority, const char *format, va_list args);
 ])
 ])
 
-m4_include([m4-sane/byteorder.m4])
+m4_include([m4/byteorder.m4])