libefi: Mark GblBootMemoryProtocol optional

GblBootMemoryProtocol is optional in both dev and prod builds, falling
back to dynamic UEFI allocations and disk reads when absent, but relied
on ProtocolInfo's default Requirement::Mandatory and logged "Required
protocol not found" when missing.

TAG=agy
CONV=b8675d7c-1192-40a2-ba55-03c4f65150b6

Bug: None
Test: ./bazel.sh test @gbl//libefi:libefi_test
Test: ./bazel.sh test @gbl//libefi:libefi_test_dev
Test: ./bazel.sh test @gbl//efi:test @gbl//efi:test_dev
Flag: EXEMPT BUGFIX
Change-Id: I121460a339ce11f6617ca4fed93f2c39f732e87c