Cleaned up include extension.

src/lib-sieve/plugins/include/cmd-import.c

src/lib-sieve/plugins/include/cmd-include.c

src/lib-sieve/plugins/include/cmd-return.c

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

src/lib-sieve/plugins/include/ext-include-binary.h

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

src/lib-sieve/plugins/include/ext-include-common.h

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

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

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

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