librem5-evk: Enable CONFIG_HARDENED_USERCOPY_FALLBACK

This fixes a config prompt after enabling CONFIG_HARDENED_USERCOPY
Signed-off-by: Guido Günther <guido.gunther@puri.sm>

arch/arm64/configs/librem5-evk_defconfig

@@ -5291,6 +5291,7 @@ CONFIG_SECURITY=y
 # CONFIG_SECURITY_PATH is not set
 CONFIG_HAVE_HARDENED_USERCOPY_ALLOCATOR=y
 CONFIG_HARDENED_USERCOPY=y
+CONFIG_HARDENED_USERCOPY_FALLBACK=y
 # CONFIG_FORTIFY_SOURCE is not set
 # CONFIG_STATIC_USERMODEHELPER is not set
 # CONFIG_SECURITY_SMACK is not set