Revert "Remove -Werror-implicit-function-declaration from Makefile"

This reverts commit 7803d1affcc5ea1581e3defc02d78835fc30b3d1.
Revert "ext4: Speed up FITRIM by recording flags in ext4_group_info"

This reverts commit 7ff520698769509482e8f820371001ae8ac3aa67.