[C++/mingw] windows-nat.c casts

[C++/mingw] windows-nat.c casts

Fixes a set of errors like:

../../src/gdb/windows-nat.c: In function 'void _initialize_loadable()':
../../src/gdb/windows-nat.c:2778:30: error: invalid conversion from 'void*' to 'BOOL (*)(DWORD) {aka int (*)(long unsigned int)}' [-fpermissive]
       DebugActiveProcessStop = (void *)
                              ^

gdb/ChangeLog:
2015-11-17  Pedro Alves  <palves@redhat.com>

	* windows-nat.c (AdjustTokenPrivileges_ftype)
	(DebugActiveProcessStop_ftype, DebugBreakProcess_ftype)
	(DebugSetProcessKillOnExit_ftype, EnumProcessModules_ftype)
	(GetCurrentConsoleFont_ftype, GetModuleInformation_ftype)
	(LookupPrivilegeValueA_ftype, OpenProcessToken_ftype)
	(GetConsoleFontSize_ftype): New typedefs.
	(AdjustTokenPrivileges, DebugActiveProcessStop)
	(DebugBreakProcess, DebugSetProcessKillOnExit, EnumProcessModules)
	(GetConsoleFontSize, GetCurrentConsoleFont, GetModuleInformation)
	(LookupPrivilegeValueA, OpenProcessToken, GetConsoleFontSize):
	Adjust.
	(GetModuleFileNameEx_ftype): New typedef.
	(GetModuleFileNameEx): Use it.
	(_initialize_loadable): Define GPA macro and use it.