src: fix unused-result warning

[santigimeno]

Checklist

• make -j4 test (UNIX), or vcbuild test nosign (Windows) passes
• commit message follows commit guidelines

Affected core subsystem(s)

c++

Description of change

I was getting these warnings on OS X locally:

../src/node_util.cc:138:43: warning: ignoring return value of function declared with warn_unused_result attribute [-Wunused-result]
PER_ISOLATE_PRIVATE_SYMBOL_PROPERTIES(V)
^~
../src/env.h:56:3: note: expanded from macro 'PER_ISOLATE_PRIVATE_SYMBOL_PROPERTIES'
V(alpn_buffer_private_symbol, "node:alpnBuffer") \
^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
../src/node_util.cc:133:3: note: expanded from macro 'V'
target->Set(context, \
^~~~~~~~~~~ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
../src/node_util.cc:138:43: warning: ignoring return value of function declared with warn_unused_result attribute [-Wunused-result]
PER_ISOLATE_PRIVATE_SYMBOL_PROPERTIES(V)
^~
../src/env.h:57:3: note: expanded from macro 'PER_ISOLATE_PRIVATE_SYMBOL_PROPERTIES'
V(arrow_message_private_symbol, "node:arrowMessage") \
^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
../src/node_util.cc:133:3: note: expanded from macro 'V'
target->Set(context, \

[jasnell]

LGTM

[addaleax]

LGTM, CI: https://ci.nodejs.org/job/node-test-commit/4959/

[Trott]

CI failures are one known flaky on Linux and one apparent CI infra failure on Raspberry Pi. Seemingly unrelated to this change.

[bnoordhuis]

LGTM but can you s/unused-result/-Wunused-result/ in the commit log?

[fhinkel]

LGTM

[cjihrig]

LGTM

[santigimeno]

Landed in cc00be6 with commit log amended

[Fishrock123]

Depends on #7082. Please mark these as dont-land-on-v6.x.