Add missing plugins.c to SRCS in util/support

Reported by Arlene Berry.

https://github.com/krb5/krb5/commit/f1ed53bad92ef369c730635d3727d277d733c539
Author: Greg Hudson <ghudson@mit.edu>
Commit: f1ed53bad92ef369c730635d3727d277d733c539
Branch: master
 src/util/support/Makefile.in |    1 +
 src/util/support/deps        |    4 ++++
 2 files changed, 5 insertions(+), 0 deletions(-)