efi_loader: fix build error for freestanding.o

Since commit f51a2264 ("efi_loader: provide freestanding library") in
parallel builds errors

    lib/efi_selftest/../efi_loader/efi_freestanding.o:
    file not recognized: File truncated

occur. Obviously make cannot correctly sequence parallel builds with a
dependency like ../efi_loader/efi_freestanding.o.

Fixes: f51a2264 ("efi_loader: provide freestanding library")
Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>