maint: fix error in syntax-check checking

maint: fix error in syntax-check checking

The previous commit introduced a bug into the following syntax-check,
and thus effectively turned it off:

  $ make sc_prohibit_test_calls_print_ver_with_irrelevant_argument; \
      echo $?
  prohibit_test_calls_print_ver_with_irrelevant_argument
  fatal: cannot change to 'grep': No such file or directory
  0

* cfg.mk (sc_prohibit_test_calls_print_ver_with_irrelevant_argument):
Remove changing directory, and pass $(srcdir) as argument to 'git -C'.