tos3to4: fix postrm script error - trying to execute luci-app-squid

The current tos3to4 package causes the following error:

    INFO:Running postrm of tos3to4
    /usr/lib/opkg/info//tos3to4.postrm: line 1: luci-app-squid: not found
    /usr/lib/opkg/info//tos3to4.postrm: line 20: syntax error: unexpected "}"
    INFO:Running postrm of cznic-repo-keys-test
    INFO:Cleaning up control files
    ERROR:Failed operations:
    tos3to4/postrm: /usr/lib/opkg/info//tos3to4.postrm: line 1: luci-app-squid: not found
    /usr/lib/opkg/info//tos3to4.postrm: line 20: syntax error: unexpected "}"

This is because of the use of backticks in the string in the shell script being interpreted as evaluation. A patch is attached; I'd send a MR but I'm not allowed to fork the repository (could I/we please get that access? it'd make submitting fixes much simpler).

fix-backticks.patch