Fix build error for sdk_gphone_x86_64-userdebug

Increases BOARD_SYSTEMIMAGE_PARTITION_RESERVED_SIZE to 128MB to fix
the build break. This is because we enabled AVB signing in the following
CL. And verified boot metadata will consume the reserved size in current
build system.

  https://android-review.googlesource.com/c/platform/build/+/745691

Bug: None
Test: build sdk_gphone_x86_64-userdebug
Change-Id: If1f44dccbc3a69b1b38a23e979f50c2c52401a25