gdb/riscv: Add back missing braces in riscv-linux-nat.c

In this commit:

    commit ee67fd7f3f6ca78eede2862e309c0bcf266bbd7e
    Date:   Thu Oct 25 12:03:31 2018 +0100

        gdb/riscv: Use correct regnum in riscv_linux_nat_target::fetch_registers

I incorrectly removed a set of braces in violation of the GDB coding
standard.  This commit adds them back.

gdb/ChangeLog:

	* riscv-linux-nat.c (riscv_linux_nat_target::fetch_registers):
	Add missing braces.  No functional change.