Core: ngx_module_t compatibility with C++.

Changes to NGX_MODULE_V1 and ngx_module_t in 85dea406e18f (1.9.11)
broke all modules written in C++, because ISO C++11 does not allow
conversion from string literal to char *.

Signed-off-by: Piotr Sikora <piotrsikora@google.com>