qemu: common: Fix build with update capsule

The common emulation Makefile has a dependency on a non-existent
qemu_capsule.o when building with support for capsule update enabled
(CONFIG_EFI_RUNTIME_UPDATE_CAPSULE=y).
The code which was in qemu_capsule.c has been completely moved to
lib/efi_loader/efi_capsule.c by commit 7a6fb28c ("efi_loader: capsule:
add back efi_get_public_key_data()").
Remove the false dependency.

This fixes the following build error:

  make[1]: *** No rule to make target 'board/emulation/common/qemu_capsule.o', needed by 'board/emulation/common/built-in.o'.  Stop.

Fixes: commit 47a25e81 ("Revert "efi_capsule: Move signature from DTB to .rodata"")
Signed-off-by: Vincent Stehlé <vincent.stehle@arm.com>
Cc: Simon Glass <sjg@chromium.org>