gcc version 10 changed the default from -fcommon to -fno-common and as a
result e2fsprogs unit tests fail because tst_libext2fs.c end up with a
build error.
Fix the problem by creating an extern declaration of said variables in
debugfs.h and just setting them in tst_libext2fs.c without additional
declaration.
Signed-off-by: Lukas Czerner <lczerner@redhat.com>
