Fixed invalid cast on mingw.

Fixes GCC error:
SkThread_win.cpp:39:61: error: invalid static_cast from type ‘int32_t {aka int}’ to type ‘const volatile LONG& {aka const volatile long int&}’

Review URL: https://codereview.chromium.org/17047004

git-svn-id: http://skia.googlecode.com/svn/trunk@9706 2bbb7eff-a529-9590-31e7-b0007b416f81
1 file changed