efi_loader: copy GUID in InstallProtocolInterface()

InstallProtocolInterface() is called with a pointer to the protocol GUID.
There is not guarantee that the memory used by the caller for the protocol
GUID stays allocated. To play it safe the GUID should be copied to U-Boot's
internal structures.

Reported-by: Joerie de Gram <j.de.gram@gmail.com>
Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>