nsis: request admin execution level

The current state of our NSIS installer doesn't support per-user
installation and needs a major rework to do so, therefore the user
is required to have admin privileges to install VLC.

Still, RequestExecutionLevel was set to 'user' to be able to drop
privileges when launching vlc.exe at the end of the installation but
this was improved since (90496f23) and
confirmed by the accesschk tool.

However requesting user execution level makes the unpacking occurs
twice: once with the user level and once after elevation.

Requesting admin permissions fixes the unpacking issue while still being
safe when launching VLC after the installation on XP and above.

extras/package/win32/NSIS/vlc.win32.nsi.in

@@ -56,7 +56,7 @@ InstType $Name_InstTypeRecommended
 InstType $Name_InstTypeMinimum
 InstType $Name_InstTypeFull
 
-RequestExecutionLevel user
+RequestExecutionLevel admin
 !addincludedir NSIS
 !addplugindir NSIS
 !include UAC.nsh