efi: loader: Make efi_runtime_mmio static

efi_runtime_mmio is only referenced in efi_boottime.c

Signed-off-by: Bin Meng <bmeng@tinylab.org>
diff --git a/lib/efi_loader/efi_runtime.c b/lib/efi_loader/efi_runtime.c
index cee96bf..bf54d6a 100644
--- a/lib/efi_loader/efi_runtime.c
+++ b/lib/efi_loader/efi_runtime.c
@@ -32,7 +32,7 @@
 };
 
 /* This list contains all runtime available mmio regions */
-LIST_HEAD(efi_runtime_mmio);
+static LIST_HEAD(efi_runtime_mmio);
 
 static efi_status_t __efi_runtime EFIAPI efi_unimplemented(void);