maint: fix sc_long_lines to warn about 100+ maximum line length again

maint: fix sc_long_lines to warn about 100+ maximum line length again

Since commit v8.23-60-g414a8cf, the above check failed to find sources
where the length of the longest line is a 3 or more digit number.

* cfg.mk (sc_long_lines): Remove the '\' escape character before '{'
and '}', because this would mean literal '{' and '}' characters in
an extended regular expression in sed(1).