When building kernel:

/usr/src/sys/dev/em/if_em.c:80:23: e1000_api.h: No such file or directory
/usr/src/sys/dev/em/if_em.c:81:25: e1000_82575.h: No such file or directory
/usr/src/sys/dev/em/if_em.c:82:19: if_em.h: No such file or directory
mkdep: compile failed
*** Error code 1

root# ls /usr/src/sys/dev/em/e1000_[a8]*
/usr/src/sys/dev/em/e1000_80003es2lan.c /usr/src/sys/dev/em/e1000_82543.h
/usr/src/sys/dev/em/e1000_80003es2lan.h /usr/src/sys/dev/em/e1000_82571.c
/usr/src/sys/dev/em/e1000_82540.c       /usr/src/sys/dev/em/e1000_82571.h
/usr/src/sys/dev/em/e1000_82541.c       /usr/src/sys/dev/em/e1000_82575.c
/usr/src/sys/dev/em/e1000_82541.h       /usr/src/sys/dev/em/e1000_82575.h
/usr/src/sys/dev/em/e1000_82542.c       /usr/src/sys/dev/em/e1000_api.c
/usr/src/sys/dev/em/e1000_82543.c       /usr/src/sys/dev/em/e1000_api.h
root# ls /usr/src/sys/dev/em/if_em.h
/usr/src/sys/dev/em/if_em.h