win: Let Doxygen see CRASHPAD_SIMULATE_CRASH()

Doxygen doesn’t generate documentation for macros unless the file that
defines it has a \file directive.

R=scottmg@chromium.org

Review URL: https://codereview.chromium.org/1406253009 .

client/simulate_crash_win.h

@@ -20,6 +20,8 @@
 #include "client/crashpad_client.h"
 #include "util/win/capture_context.h"
 
+//! \file
+
 //! \brief Captures the CPU context and captures a dump without an exception.
 #define CRASHPAD_SIMULATE_CRASH()                        \
   do {                                                   \