fix error: redefinition of ‘skb_frag_off’ in kcompat.h for Ubuntu 18.04 kernel 4.15.0-189-generic

[azdolinski]

prevent duplicated skb_frag_off for Ubuntu 18.04 kernel 4.15.0-189-generic
propose of resolution for issue #12

tested:

root@az:/var/lib/dkms/e1000e/3.8.7/build/src# make
*** The target kernel has CONFIG_MODULE_SIG_ALL enabled, but
*** the signing key cannot be found. Module signing has been
*** disabled for this build.
make[1]: Entering directory '/usr/src/linux-headers-4.15.0-189-generic'
  CC [M]  /var/lib/dkms/e1000e/3.8.7/build/src/netdev.o
  CC [M]  /var/lib/dkms/e1000e/3.8.7/build/src/ethtool.o
  CC [M]  /var/lib/dkms/e1000e/3.8.7/build/src/ich8lan.o
  CC [M]  /var/lib/dkms/e1000e/3.8.7/build/src/mac.o
  CC [M]  /var/lib/dkms/e1000e/3.8.7/build/src/nvm.o
  CC [M]  /var/lib/dkms/e1000e/3.8.7/build/src/phy.o
  CC [M]  /var/lib/dkms/e1000e/3.8.7/build/src/manage.o
  CC [M]  /var/lib/dkms/e1000e/3.8.7/build/src/80003es2lan.o
  CC [M]  /var/lib/dkms/e1000e/3.8.7/build/src/82571.o
  CC [M]  /var/lib/dkms/e1000e/3.8.7/build/src/param.o
  CC [M]  /var/lib/dkms/e1000e/3.8.7/build/src/ptp.o
  CC [M]  /var/lib/dkms/e1000e/3.8.7/build/src/kcompat.o
  LD [M]  /var/lib/dkms/e1000e/3.8.7/build/src/e1000e.o
  Building modules, stage 2.
  MODPOST 1 modules
  CC      /var/lib/dkms/e1000e/3.8.7/build/src/e1000e.mod.o
  LD [M]  /var/lib/dkms/e1000e/3.8.7/build/src/e1000e.ko
make[1]: Leaving directory '/usr/src/linux-headers-4.15.0-189-generic'
root@az:/var/lib/dkms/e1000e/3.8.7/build/src# uname -a
Linux az 4.15.0-189-generic #200-Ubuntu SMP Wed Jun 22 19:53:37 UTC 2022 x86_64 x86_64 x86_64 GNU/Linux