cmake: move OUTPUT argument in the add_custom_command() line

[vszakats]

For greppability.

---

docs/CMakeLists.txt:    add_custom_command(OUTPUT "${_man_target}"
docs/cmdline-opts/CMakeLists.txt:add_custom_command(OUTPUT "${CURL_MANPAGE}" "${CURL_ASCIIPAGE}"
docs/libcurl/CMakeLists.txt:      add_custom_command(OUTPUT ${_rofffiles}
docs/libcurl/CMakeLists.txt:add_custom_command(OUTPUT "libcurl-symbols.md"
scripts/CMakeLists.txt:      add_custom_command(OUTPUT "${_completion_fish}"
scripts/CMakeLists.txt:      add_custom_command(OUTPUT "${_completion_zsh}"
src/CMakeLists.txt:  add_custom_command(OUTPUT "tool_hugehelp.c"
src/CMakeLists.txt:    add_custom_command(OUTPUT "tool_ca_embed.c"
tests/certs/CMakeLists.txt:add_custom_command(OUTPUT ${GENERATEDCERTS}
tests/client/CMakeLists.txt:add_custom_command(OUTPUT "${BUNDLE_SRC}"
tests/libtest/CMakeLists.txt:add_custom_command(OUTPUT "lib1521.c"
tests/libtest/CMakeLists.txt:add_custom_command(OUTPUT "${BUNDLE_SRC}"
tests/server/CMakeLists.txt:add_custom_command(OUTPUT "${BUNDLE_SRC}"
tests/tunit/CMakeLists.txt:add_custom_command(OUTPUT "${BUNDLE_SRC}"
tests/unit/CMakeLists.txt:add_custom_command(OUTPUT "${BUNDLE_SRC}"