btrfs-progs: adjust install-static patch to install headers under $(prefix)/include...

btrfs-progs: adjust install-static patch to install headers under $(prefix)/include/btrfs

Fixes:
http://autobuild.buildroot.net/results/15cc6523a42bf66508b2b37fd1fcb74625561ec2/

btrfs-progs 'make install' installs headers under $(prefix)/include/btrfs,
but our patch to also install headers for 'install-static' installed
directly into $(prefix)/include, confusing other packages as btrfs-progs
headers have very common names such as version, sizes.h, list.h, ioctl.h, ..

Fix it by adjusting the patch to also install under $(prefix)/include/btrfs.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>