This is part of the kernel source code, so you most likely did not install it by default.
Here you will find out which package belongs to the file:
~$ dpkg -S skbuff.h
linux-headers-2.6.32-22: /usr/src/linux-headers-2.6.32-22/include/linux/skbuff.h
linux-headers-2.6.32-22-generic: /usr/src/linux-headers-2.6.32-22-generic/include/linux/skbuff.h
linux-headers-2.6.32-23: /usr/src/linux-headers-2.6.32-23/include/linux/skbuff.h
linux-headers-2.6.32-23-generic: /usr/src/linux-headers-2.6.32-23-generic/include/linux/skbuff.h
linux-headers-2.6.32-24: /usr/src/linux-headers-2.6.32-24/include/linux/skbuff.h
linux-headers-2.6.32-24-generic: /usr/src/linux-headers-2.6.32-24-generic/include/linux/skbuff.h
So, you just have to install the required package to add the file to your computer.
On Ubuntu, you would do:
$ sudo apt install linux-headers-generic
The final -genericis a meta package that will automatically update your headlines as new ones arrive. See also this answer by Ubuntu .