riscv: Fixed kernel test robot warning

Kernel test robot throws below warning -

   arch/riscv/kernel/asm-offsets.c:14:6: warning: no previous prototype
for 'asm_offsets' [-Wmissing-prototypes]
      14 | void asm_offsets(void)
         |      ^~~~~~~~~~~

This patch should fixed it.
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Souptick Joarder <jrdr.linux@gmail.com>
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>

arch/riscv/kernel/asm-offsets.c

@@ -11,6 +11,8 @@
 #include <asm/thread_info.h>
 #include <asm/ptrace.h>
 
+void asm_offsets(void);
+
 void asm_offsets(void)
 {
 	OFFSET(TASK_THREAD_RA, task_struct, thread.ra);