riscv: Avoid kgdb.h including gdb_xml.h to solve unused-const-variable warning

The constant arrays in gdb_xml.h are only used in arch/riscv/kernel/kgdb.c,
but other c files may include the gdb_xml.h indirectly via including the
kgdb.h. Hence, It will cause many unused-const-variable warnings. This
patch makes the kgdb.h not to include the gdb_xml.h to solve this problem.

Signed-off-by: Vincent Chen <vincent.chen@sifive.com>
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>