riscv: Fix build warning for mm/init

Add static keyword for resource_init, this function is only used in this
object file.
Signed-off-by: Zong Li <zong.li@sifive.com>
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>

arch/riscv/mm/init.c

@@ -517,7 +517,7 @@ void mark_rodata_ro(void)
 }
 #endif
 
-void __init resource_init(void)
+static void __init resource_init(void)
 {
 	struct memblock_region *region;