qga: fix missing static and prototypes windows warnings

qga: fix missing static and prototypes windows warnings

Reported by clang++, but not by g++.

../qga/vss-win32/provider.cpp:48:6: error: no previous prototype for function 'LockModule' [-Werror,-Wmissing-prototypes]
   48 | void LockModule(BOOL lock)
      |      ^
../qga/vss-win32/provider.cpp:48:1: note: declare 'static' if the function is not intended to be used outside of this translation unit
   48 | void LockModule(BOOL lock)
      | ^
      | static
../qga/vss-win32/provider.cpp:531:13: error: no previous prototype for function 'DllMain' [-Werror,-Wmissing-prototypes]
  531 | BOOL WINAPI DllMain(HINSTANCE hinstDll, DWORD dwReason, LPVOID lpReserved)
      |             ^
../qga/vss-win32/provider.cpp:531:1: note: declare 'static' if the function is not intended to be used outside of this translation unit
  531 | BOOL WINAPI DllMain(HINSTANCE hinstDll, DWORD dwReason, LPVOID lpReserved)
      | ^
      | static

Signed-off-by: Pierrick Bouvier <pierrick.bouvier@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Konstantin Kostiuk <kkostiuk@redhat.com>
Link: https://lore.kernel.org/r/20241031040426.772604-7-pierrick.bouvier@linaro.org
Signed-off-by: Konstantin Kostiuk <kkostiuk@redhat.com>