bootmeth: efi: Make distro_efi_boot() static

As it is only called in bootmeth_efi.c

Signed-off-by: Bin Meng <bmeng@tinylab.org>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
diff --git a/boot/bootmeth_efi.c b/boot/bootmeth_efi.c
index 1c9f2b1..ae936c8 100644
--- a/boot/bootmeth_efi.c
+++ b/boot/bootmeth_efi.c
@@ -409,7 +409,7 @@
 	return 0;
 }
 
-int distro_efi_boot(struct udevice *dev, struct bootflow *bflow)
+static int distro_efi_boot(struct udevice *dev, struct bootflow *bflow)
 {
 	ulong kernel, fdt;
 	char cmd[50];