Variables extension: removed public dependency on ext-variables-limits.h.

src/lib-sieve/plugins/include/ext-include-variables.c

src/lib-sieve/plugins/variables/cmd-set.c

src/lib-sieve/plugins/variables/ext-variables-arguments.c

src/lib-sieve/plugins/variables/ext-variables-common.c

src/lib-sieve/plugins/variables/ext-variables-limits.h

src/lib-sieve/plugins/variables/ext-variables-name.c

src/lib-sieve/plugins/variables/ext-variables-operands.c

src/lib-sieve/plugins/variables/sieve-ext-variables.h