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>

arch/riscv/include/asm/gdb_xml.h

@@ -3,8 +3,7 @@
 #ifndef __ASM_GDB_XML_H_
 #define __ASM_GDB_XML_H_
 
-#define kgdb_arch_gdb_stub_feature riscv_gdb_stub_feature
-static const char riscv_gdb_stub_feature[64] =
+const char riscv_gdb_stub_feature[64] =
 			"PacketSize=800;qXfer:features:read+;";
 
 static const char gdb_xfer_read_target[31] = "qXfer:features:read:target.xml:";

arch/riscv/include/asm/kgdb.h

@@ -105,7 +105,9 @@ static inline void arch_kgdb_breakpoint(void)
 #define DBG_REG_BADADDR_OFF 34
 #define DBG_REG_CAUSE_OFF 35
 
-#include <asm/gdb_xml.h>
+extern const char riscv_gdb_stub_feature[64];
+
+#define kgdb_arch_gdb_stub_feature riscv_gdb_stub_feature
 
 #endif
 #endif