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>

src/core/ngx_module.h

diff --git a/src/core/ngx_module.h b/src/core/ngx_module.h
index cd28c49..3e74def 100644
--- a/src/core/ngx_module.h
+++ b/src/core/ngx_module.h
@@ -253,7 +253,7 @@
     ngx_uint_t            spare1;
 
     ngx_uint_t            version;
-    char                 *signature;
+    const char           *signature;
 
     void                 *ctx;
     ngx_command_t        *commands;