Fix build with -Werror=calloc-transposed-args

[mrc0mmand]

Fix build with -Werror=calloc-transposed-args

calloc() expects the number of elements as the first argument, and the
size of each element as the second one. Recent-enough gcc/clang started
warning about this, so let's fix the order of the arugments to avoid
build errors with -Werror.

[9/30] Compiling C object dfuzzer.p/src_suppression.c.o
../src/suppression.c: In function ‘df_suppression_load’:
../src/suppression.c:123:37: warning: ‘calloc’ sizes specified with ‘sizeof’ in the earlier argument and not in the later argument [-Wcalloc-transposed-args]
  123 |                 item = calloc(sizeof(*item), 1);
      |                                     ^
../src/suppression.c:123:37: note: earlier argument should specify number of elements, later size of each element

Resolves #143