riscv: lib: modify the indent

We usually use a space in function declaration, rather than a tab.

Signed-off-by: Zong Li <zong.li@sifive.com>
Reviewed-by: Sean Anderson <seanga2@gmail.com>
Reviewed-by: Rick Chen <rick@andestech.com>

arch/riscv/include/asm/cache.h

@@ -8,7 +8,7 @@
 #define _ASM_RISCV_CACHE_H
 
 /* cache */
-void	cache_flush(void);
+void cache_flush(void);
 
 /*
  * The current upper bound for RISCV L1 data cache line sizes is 32 bytes.