Variables: fixed :count issue for the string test.

Makefile.am

TODO

src/lib-sieve/mcht-contains.c

src/lib-sieve/mcht-is.c

src/lib-sieve/mcht-matches.c

src/lib-sieve/plugins/regex/mcht-regex.c

src/lib-sieve/plugins/relational/mcht-count.c

src/lib-sieve/plugins/relational/mcht-value.c

src/lib-sieve/plugins/variables/tst-string.c

src/lib-sieve/sieve-match-types.h

src/lib-sieve/sieve-match.c

tests/extensions/variables/string.svtest