pkg: include missing <string.h>

My local environment seems to be seeing some pollution; we need
<string.h> for strlen.

PR: 284021
Fixes: 2e065d74a5b0e ("pkg: add a pkgsign_verify_data [...]")

(cherry picked from commit b8770ce1dfed52fcb7249cdf3cf4d4d16357b9fd)