Fix ENABLE_UNRAR=off build

[ragusaa]

Cmake errors out when the ENABLE_UNRAR=off option is used. This commit
addresses that.

[micahsnyder]

This fixes #225

[micahsnyder]

Also cherry-picked to rel/0.104 so it gets into RC2 and release.

[DanielRuf]

Not sure if related but on current main branch I get this when I compile with this setting:

/workspaces/clamav/common/cert_util.c:42:16: warning: variable ‘ret’ set but not used [-Wunused-but-set-variable]
   42 |     cl_error_t ret = CL_EFORMAT;
      |                ^~~
[ 69%] Building C object common/CMakeFiles/common.dir/actions.c.o
In file included from /workspaces/clamav/libclamav/matcher.h:29,
                 from /workspaces/clamav/libclamav/others.h:22,
                 from /workspaces/clamav/common/actions.c:52:
/workspaces/clamav/libclamav/others.h:57:10: fatal error: unrar_iface.h: No such file or directory
   57 | #include "unrar_iface.h"
      |          ^~~~~~~~~~~~~~~
compilation terminated.
make[2]: *** [common/CMakeFiles/common.dir/build.make:76: common/CMakeFiles/common.dir/actions.c.o] Error 1
make[1]: *** [CMakeFiles/Makefile2:1662: common/CMakeFiles/common.dir/all] Error 2
make: *** [Makefile:163: all] Error 2

cmake .. -D CMAKE_BUILD_TYPE=RelWithDebInfo -D CMAKE_INSTALL_PREFIX=/usr -D CMAKE_INSTALL_LIBDIR=/usr/lib -D APP_CONFIG_DIRECTORY=/etc/clamav -D DATABASE_DIRECTORY=/workspaces/clamav/scan-rules -D ENABLE_STATIC_LIB=ON -D ENABLE_SHARED_LIB=OFF -D ENABLE_UNRAR=OFF