Make doesn't detect changes in the header files

[nicolaskribas]

$ make bootstrap
make: 'bootstrap' is up to date.

$ touch bootstrap.bpf.c

$ make bootstrap
  BPF      .output/bootstrap.bpf.o
  GEN-SKEL .output/bootstrap.skel.h
  CC       .output/bootstrap.o
  BINARY   bootstrap

$ touch bootstrap.c

$ make bootstrap
  CC       .output/bootstrap.o
  BINARY   bootstrap

# working as expected till now

$ touch bootstrap.h

$ make bootstrap
make: 'bootstrap' is up to date.

# change in bootstrap.h doesn't trigger a rebuild