Fixes the following build error with GCC 10:
/usr/bin/ld: read_fs.o:(.bss+0x0): multiple definition of `swap'; mksquashfs.o:(.bss+0x1b2a88): first defined here
And a compile warning.
Fixes: FS#3104, FS#3119
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
(cherry picked from commit 1bbc1aa884)
