fix qa warning:

/usr/share/applications/nvclock.desktop: error: (will be fatal in the future):
value "nvclock.png" for key "Icon" in group "Desktop Entry" is an icon name
with an extension, but there should be no extension as described in the Icon
Theme Specification if the value is not an absolute path

--- a/nvclock.desktop
+++ b/nvclock.desktop
@@ -3,7 +3,7 @@ Encoding=UTF-8
 Name=NVCLOCK
 Comment=Overclock and Monitor NVIDIA cards
 Exec=nvclock_gtk
-Icon=nvclock.png
+Icon=nvclock
 Terminal=false
 Type=Application
 Categories=System;