CI: suppress -Wmissing-braces in CentOS 7 CMake build

CI: suppress -Wmissing-braces in CentOS 7 CMake build

The compiler on CentOS 7 is an older version of GCC, 4.8.5, which spuriously
issues warnings for code like `foo_t x = {0}` where `foo_t` is a struct
containing an aggregate (array, struct, union). This form of initialization is
meant to be valid for all structs in C99. We need to suppress this to avoid CI
warnings that fail the build in an upcoming commit.

Gitlab: #2302