riscv: Add missing includes for DDB

The #ifdef DDB code in parse_metadata was dead code without opt_ddb.h.
While here, update the call to db_fetch_ksymtab for changes in commit
02bc014a200a.

Reviewed by: mhorne
Obtained from: CheriBSD
Differential Revision: https://reviews.freebsd.org/D43919

(cherry picked from commit 962b0bcbd924d308016237abc991280f15777e7f)