FreeBSD/src b8770ceusr.sbin/pkg rsa.c

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 [...]")
MFC after:      immediately (trivial build fix)
DeltaFile
+1-0usr.sbin/pkg/rsa.c
+1-01 files

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