fastboot: only require Linux modules on Linux

The following three modules aren't defined for Windows
builds:
 e2fsdroid mke2fs.conf sload_f2fs

Which leads to the following errors:
build/make/core/main.mk:586: warning: Missing required dependency host_cross_e2fsdroid from module host_cross_fastboot defined in system/core/fastboot/Android.mk
build/make/core/main.mk:586: warning: Missing required dependency host_cross_mke2fs.conf from module host_cross_fastboot defined in system/core/fastboot/Android.mk
build/make/core/main.mk:586: warning: Missing required dependency host_cross_sload_f2fs from module host_cross_fastboot defined in system/core/fastboot/Android.mk

Here, I've removed their requirement for the Windows build and
only included the requirement for the Linux build.

Bug: 7456955
Test: above errors no longer present
Change-Id: I14fdb880d868f893fa5899ea200820bd885a3bc1