ANDROID: gki_defconfig: CONFIG_MMC_BLOCK=m

Align x86 gki config with arm64 gki config w.r.t. MMC_BLOCK.

Signed-off-by: Mark Salyzyn <salyzyn@google.com>
Test: compile
Bug: 140652382
Change-Id: Ic343c0157b7604a8334a44dea25c0a50a1fb668e