riscv: correct symbol version of __vdso_flush_icache

Previously, __riscv_flush_icache would not work correctly as
__vdso_flush_icache had a wrong symbol version. Fix this by correcting
symbol version.

Fixes: 0a48860c27 ("add riscv64 architecture support")

src/linux/cache.c

@@ -21,7 +21,7 @@ weak_alias(__cachectl, cachectl);
 #ifdef SYS_riscv_flush_icache
 
 #define VDSO_FLUSH_ICACHE_SYM "__vdso_flush_icache"
-#define VDSO_FLUSH_ICACHE_VER "LINUX_4.5"
+#define VDSO_FLUSH_ICACHE_VER "LINUX_4.15"
 
 static void *volatile vdso_func;