riscv: Compile instr_size.c into the kernel when DTrace is configured

Reported by:	Jenkins
Fixes:	080e56a6c9 ("dtrace: expose dtrace_instr_size() to userland and implement it for riscv")

sys/conf/files.riscv

@@ -1,6 +1,7 @@
 # $FreeBSD$
 cddl/dev/dtrace/riscv/dtrace_asm.S			optional dtrace compile-with "${DTRACE_S}"
 cddl/dev/dtrace/riscv/dtrace_subr.c			optional dtrace compile-with "${DTRACE_C}"
+cddl/dev/dtrace/riscv/instr_size.c			optional dtrace compile-with "${DTRACE_C}"
 cddl/dev/fbt/riscv/fbt_isa.c				optional dtrace_fbt | dtraceall compile-with "${FBT_C}"
 crypto/des/des_enc.c		optional	netsmb
 dev/ofw/ofw_cpu.c		optional	fdt