ddb: Fix format string errors in db_pprint.c

For some reason, db_expr_t is defined as "long" on 64-bit platforms and
"int" on others. When printing values of this type, simply cast them to
long to suppress compilation errors on 32-bit systems.

Reviewed by: bnovkov
Fixes: c21bc6f3c242 ("ddb: Add CTF-based pretty printing")