Suppress a warning in ruby/win32.h [Fix GH-1591]

Fix a warning in ruby/win32.h which can cause failures with mkmf

The return value is implicit type casted from 'long double' to 'double', currently.
This causes a gcc warning like this:

```
In file included from C:\Ruby24-x64\include\ruby-2.4.0/ruby/defines.h:243:0,
                 from C:\Ruby24-x64\include\ruby-2.4.0/ruby/ruby.h:36,
                 from C:\Ruby24-x64\include\ruby-2.4.0/ruby.h:33,
                 from conftest.c:1:
C:\Ruby24-x64\include\ruby-2.4.0/ruby/win32.h: In function 'rb_w32_pow':
C:\Ruby24-x64\include\ruby-2.4.0/ruby/win32.h:786:12: warning: conversion to 'double' from 'long double' may alter its value [-Wfloat-conversion]
     return powl(x, y);
            ^~~~~~~~~~
```

This is fixed by the attached explicit type cast.

Moreover when CFLAGS is set to '-Wconversion', it prevents the compiler from
building. This is the case at the nokogiri gem.

The original issue arose at RubyInstaller2: 576a0eb70a

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@58423 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

include/ruby/win32.h

@@ -781,7 +781,7 @@ RUBY_SYMBOL_EXPORT_END
 static inline double
 rb_w32_pow(double x, double y)
 {
-    return powl(x, y);
+    return (double)powl(x, y);
 }
 #elif defined(__MINGW64_VERSION_MAJOR)
 double rb_w32_pow(double x, double y);