package/btrfs-progs: fix build with kernel < 5.11

package/btrfs-progs: fix build with kernel < 5.11

Fix the following build failure with kernel < 5.11 raised since commit
f363bc2958812539cf248f6cf44fe519445adf7a:

/tmp/instance-1/output-1/host/opt/ext-toolchain/bin/../lib/gcc/powerpc64-buildroot-linux-gnu/9.3.0/../../../../powerpc64-buildroot-linux-gnu/bin/ld: kernel-shared/volumes.o: in function `create_chunk':
volumes.c:(.text+0x17f8): undefined reference to `__ALIGN_KERNEL'

Fixes:
 - http://autobuild.buildroot.org/results/b630dbd078656a974c6ac9645b2e87279ef2adc0

Signed-off-by: Fabrice Fontaine <[email protected]>
Signed-off-by: Thomas Petazzoni <[email protected]>