WIN32: fix calling conventions for 32-bit builds

On 32-bit Windows Intel builds the __cdecl and __stdcall calling
conventions are different so labeling the functions that are
exported or assigned to function pointers matters.

Change-Id: I03b6f34baeb9ffb2e683fd979f12f27a5078a4da

kdc/windc.c

@@ -218,7 +218,7 @@ _kdc_check_access(krb5_context context,
     return ret;
 }
 
-uintptr_t
+uintptr_t KRB5_CALLCONV
 kdc_get_instance(const char *libname)
 {
     static const char *instance = "libkdc";