Clang -Wcast-function-type-mismatch warning fix

[ajkhoury]

This fixes a -Wcast-function-type-mismatch warning that is generated when compiling examples using Clang on Windows:

.../xed/wkit/examples/xed-util-disas-pecoff.cpp:87:8: error: cast from 'FARPROC' (aka 'long long (*)()') to 'fptr_t' (aka 'int (*)(void *)') converts to incompatible function type [-Werror,-Wcast-function-type-mismatch]
   87 |    p = (fptr_t) GetProcAddress(
      |        ^~~~~~~~~~~~~~~~~~~~~~~~
   88 |                    GetModuleHandle(TEXT("kernel32.dll")), 
      |                    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
   89 |                    function_name);
      |                    ~~~~~~~~~~~~~~
1 error generated.

This causes a compiler error due to -Werror being set by default. This commit adds a void pointer cast before casting to fptr_t which effectively ignores this warning.