efi/libstub: Fix missing-prototype warning for skip_spaces()

Include <linux/string.h> into skip_spaces.c to silence a compiler
warning about a missing prototype.
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>

drivers/firmware/efi/libstub/skip_spaces.c

 // SPDX-License-Identifier: GPL-2.0
 
 #include <linux/ctype.h>
+#include <linux/string.h>
 #include <linux/types.h>
 
 char *skip_spaces(const char *str)