Fix depr.c.headers/inttypes_h.pass.cpp regression after cfa0b15c09.

cfa0b15c09 tried to disable wcstoimax in C99 mode, but accidentally
disabled it in C++11 mode too. Fix this, to make inttypes.h.pass.cpp
work again.

Change-Id: I81cbf7a855be4d9b87b8532740c2afe4a80887b4