Fix compilation for GCC 4.

src/lib-sieve/sieve-error.c

src/lib-sieve/sieve-generator.c

src/lib-sieve/sieve-interpreter.c

src/lib-sieve/sieve-lexer.c

src/lib-sieve/sieve-parser.c

src/lib-sieve/sieve-result.c

src/lib-sieve/sieve-validator.c