package/exim: add libxcrypt optional dependency

package/exim: add libxcrypt optional dependency

Fix the following build failure raised since bump of glibc to version
2.39 in commit b5680f53d60acf8ff6010082f873438a39bd5d97:

crypt16.c:45:10: fatal error: crypt.h: No such file or directory
   45 | #include <crypt.h>
      |          ^~~~~~~~~

Fixes: b5680f53d60acf8ff6010082f873438a39bd5d97
 - http://autobuild.buildroot.org/results/3c846c763a05f2569db7a019af8b91b0563c8f42

Signed-off-by: Fabrice Fontaine <[email protected]>
Reviewed-by: Luca Ceresoli <[email protected]>
Signed-off-by: Yann E. MORIN <[email protected]>