wangle: make new pragma use portable to older clang

wangle: make new pragma use portable to older clang

Summary:
* folly/wangle/channel/test/MockChannelHandler.h:
Avoiding a new warning that rendered this file uncompilable
by clang-3.5 and older.  Add cpp conditionals to protect
against that.

Test Plan:
Ensure it works with clang-3.4, 3.5 and clang:dev.
I.e., ensure that this prints PASS at the end.

for i in '' 3.5 dev; do
test -n "$i" && i=--with-project-version=clang:$i
fbconfig -r --clang $i folly/wangle/channel/test && fbmake runtests_dbgo
done && echo PASS

Reviewed By: meyering@fb.com

Subscribers: mpawlowski, fugalh, mathieubaudet, folly-diffs@, jsedgwick, yfeldblum

FB internal diff: D1849474

Tasks: 6244745

Blame Revision: D1848330
@build-break